In this atomic age, we can't trust the plane->fb pointer anymore.
It might get update too late. Instead we are supposed to use the
plane_state->fb pointer instead. Let's do that in
intel_plane_obj_offset() and avoid problems from dereferencing the
potentially stale plane->fb pointer.

Paulo found this with 'kms_frontbuffer_tracking --show-hidden --run-subtest nop-1p-rte'
but it can be reproduced with just plain old kms_setplane.

I was too lazy to bisect this, so not sure exactly when it broke. The
most obvious candidate
commit ce7f1728 ("drm/i915: Fix i915_ggtt_view_equal to handle rotation correctly")
was actually still fine, so it must have broken some time after that.

Here's the resulting fireworks:
BUG: unable to handle kernel NULL pointer dereference at           (null)
IP: [<ffffffffa02d2d9a>] intel_fill_fb_ggtt_view+0x1b/0x15a [i915]
PGD 8a5f6067 PUD 8a5f5067 PMD 0
Oops: 0000 [#1] PREEMPT SMP
Modules linked in: i915 i2c_algo_bit drm_kms_helper syscopyarea sysfillrect sysimgblt fb_sys_fops drm intel_gtt agpgart netconsole mousedev hid_generic psmouse usbhid atkbd libps2 coretemp hwmon efi_pstore intel_rapl iosf_mbi x86_pkg_temp_thermal efivars pcspkr e1000e sdhci_pci ptp pps_core sdhci i2c_i801 mmc_core i2c_hid hid i8042 serio evdev sch_fq_codel ip_tables x_tables ipv6 autofs4
CPU: 1 PID: 260 Comm: kms_plane Not tainted 4.4.0-skl+ #171
Hardware name: Intel Corporation Skylake Client platform/Skylake Y LPDDR3 RVP3, BIOS SKLSE2R1.R00.B104.B00.1511030553 11/03/2015
task: ffff88008bde2d80 ti: ffff88008a6ec000 task.ti: ffff88008a6ec000
RIP: 0010:[<ffffffffa02d2d9a>]  [<ffffffffa02d2d9a>] intel_fill_fb_ggtt_view+0x1b/0x15a [i915]
RSP: 0018:ffff88008a6efa10  EFLAGS: 00010086
RAX: 0000000000000001 RBX: ffff8801674f4240 RCX: 0000000000000014
RDX: ffff88008a7440c0 RSI: 0000000000000000 RDI: ffff88008a6efa40
RBP: ffff88008a6efa30 R08: ffff88008bde3598 R09: 0000000000000001
R10: ffff88008b782000 R11: 0000000000000000 R12: 0000000000000000
R13: ffff88008a7440c0 R14: 0000000000000000 R15: ffff88008a7449c0
FS:  00007fa0c07a28c0(0000) GS:ffff88016ec40000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 000000008a6ff000 CR4: 00000000003406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Stack:
 ffff8801674f4240 0000000000000000 ffff88008a7440c0 0000000000000000
 ffff88008a6efaa0 ffffffffa02daf25 ffffffff814ec80e 0000000000070298
 ffff8800850d0000 ffff88008a6efaa0 ffffffffa02c49c2 0000000000000002
Call Trace:
 [<ffffffffa02daf25>] intel_plane_obj_offset+0x2d/0xa9 [i915]
 [<ffffffff814ec80e>] ? _raw_spin_unlock_irqrestore+0x4b/0x60
 [<ffffffffa02c49c2>] ? gen9_write32+0x2e8/0x3b8 [i915]
 [<ffffffffa02eecfc>] skl_update_plane+0x203/0x4c5 [i915]
 [<ffffffffa02ca1ab>] intel_plane_atomic_update+0x53/0x6a [i915]
 [<ffffffffa02494a4>] drm_atomic_helper_commit_planes_on_crtc+0x142/0x1d5 [drm_kms_helper]
 [<ffffffffa02de44b>] intel_atomic_commit+0x1262/0x1350 [i915]
 [<ffffffffa024a0ee>] ? __drm_atomic_helper_crtc_duplicate_state+0x2f/0x41 [drm_kms_helper]
 [<ffffffffa01ef089>] ? drm_atomic_check_only+0x3e3/0x552 [drm]
 [<ffffffffa01ef245>] drm_atomic_commit+0x4d/0x52 [drm]
 [<ffffffffa024996b>] drm_atomic_helper_update_plane+0xcb/0x118 [drm_kms_helper]
 [<ffffffffa01e42e8>] __setplane_internal+0x1c8/0x224 [drm]
 [<ffffffffa01e477f>] drm_mode_setplane+0x14e/0x172 [drm]
 [<ffffffffa01d8117>] drm_ioctl+0x265/0x3ad [drm]
 [<ffffffffa01e4631>] ? drm_mode_cursor_common+0x158/0x158 [drm]
 [<ffffffff810d00ab>] ? current_kernel_time64+0x5e/0x98
 [<ffffffff810a76ea>] ? trace_hardirqs_on_caller+0x17a/0x196
 [<ffffffff8119880f>] do_vfs_ioctl+0x42b/0x4ea
 [<ffffffff811a2b72>] ? __fget_light+0x4d/0x71
 [<ffffffff81198911>] SyS_ioctl+0x43/0x61
 [<ffffffff814ed057>] entry_SYSCALL_64_fastpath+0x12/0x6f

Cc: drm-intel-fixes@lists.freedesktop.org
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Testcase: igt/kms_plane
Reported-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453220597-28973-1-git-send-email-ville.syrjala@linux.intel.comReviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>