The last part of drm_atomic_check_only is testing whether we need to
fail with -EINVAL when modeset is not allowed, but forgets to return
the value when atomic_check() fails first.

This results in -EDEADLK being replaced by -EINVAL, and the sanity
check in drm_modeset_drop_locks kicks in:

[  308.531734] ------------[ cut here ]------------
[  308.531791] WARNING: CPU: 0 PID: 1886 at drivers/gpu/drm/drm_modeset_lock.c:217 drm_modeset_drop_locks+0x33/0xc0 [drm]
[  308.531828] Modules linked in:
[  308.532050] CPU: 0 PID: 1886 Comm: kms_atomic Tainted: G     U  W 4.13.0-rc5-patser+ #5225
[  308.532082] Hardware name: NUC5i7RYB, BIOS RYBDWi35.86A.0246.2015.0309.1355 03/09/2015
[  308.532124] task: ffff8800cd9dae00 task.stack: ffff8800ca3b8000
[  308.532168] RIP: 0010:drm_modeset_drop_locks+0x33/0xc0 [drm]
[  308.532189] RSP: 0018:ffff8800ca3bf980 EFLAGS: 00010282
[  308.532211] RAX: dffffc0000000000 RBX: ffff8800ca3bfaf8 RCX: 0000000013a171e6
[  308.532235] RDX: 1ffff10019477f69 RSI: ffffffffa8ba4fa0 RDI: ffff8800ca3bfb48
[  308.532258] RBP: ffff8800ca3bf998 R08: 0000000000000000 R09: 0000000000000003
[  308.532281] R10: 0000000079dbe066 R11: 00000000f760b34b R12: 0000000000000001
[  308.532304] R13: dffffc0000000000 R14: 00000000ffffffea R15: ffff880096889680
[  308.532328] FS:  00007ff00959cec0(0000) GS:ffff8800d4e00000(0000) knlGS:0000000000000000
[  308.532359] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  308.532380] CR2: 0000000000000008 CR3: 00000000ca2e3000 CR4: 00000000003406f0
[  308.532402] Call Trace:
[  308.532440]  drm_mode_atomic_ioctl+0x19fa/0x1c00 [drm]
[  308.532488]  ? drm_atomic_set_property+0x1220/0x1220 [drm]
[  308.532565]  ? avc_has_extended_perms+0xc39/0xff0
[  308.532593]  ? lock_downgrade+0x610/0x610
[  308.532640]  ? drm_atomic_set_property+0x1220/0x1220 [drm]
[  308.532680]  drm_ioctl_kernel+0x154/0x1a0 [drm]
[  308.532755]  drm_ioctl+0x624/0x8f0 [drm]
[  308.532858]  ? drm_atomic_set_property+0x1220/0x1220 [drm]
[  308.532976]  ? drm_getunique+0x210/0x210 [drm]
[  308.533061]  do_vfs_ioctl+0xd92/0xe40
[  308.533121]  ? ioctl_preallocate+0x1b0/0x1b0
[  308.533160]  ? selinux_capable+0x20/0x20
[  308.533191]  ? do_fcntl+0x1b1/0xbf0
[  308.533219]  ? kasan_slab_free+0xa2/0xb0
[  308.533249]  ? f_getown+0x4b/0xa0
[  308.533278]  ? putname+0xcf/0xe0
[  308.533309]  ? security_file_ioctl+0x57/0x90
[  308.533342]  SyS_ioctl+0x4e/0x80
[  308.533374]  entry_SYSCALL_64_fastpath+0x18/0xad
[  308.533405] RIP: 0033:0x7ff00779e4d7
[  308.533431] RSP: 002b:00007fff66a043d8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
[  308.533481] RAX: ffffffffffffffda RBX: 000000e7c7ca5910 RCX: 00007ff00779e4d7
[  308.533560] RDX: 00007fff66a04430 RSI: 00000000c03864bc RDI: 0000000000000003
[  308.533608] RBP: 00007ff007a5fb00 R08: 000000e7c7ca4620 R09: 000000e7c7ca5e60
[  308.533647] R10: 0000000000000001 R11: 0000000000000246 R12: 0000000000000070
[  308.533685] R13: 0000000000000000 R14: 0000000000000000 R15: 000000e7c7ca5930
[  308.533770] Code: ff df 55 48 89 e5 41 55 41 54 53 48 89 fb 48 83 c7
50 48 89 fa 48 c1 ea 03 80 3c 02 00 74 05 e8 94 d4 16 e7 48 83 7b 50 00
74 02 <0f> ff 4c 8d 6b 58 48 b8 00 00 00 00 00 fc ff df 4c 89 ea 48 c1
[  308.534086] ---[ end trace 77f11e53b1df44ad ]---

Solve this by adding the missing return.

This is also a bugfix because we could end up rejecting updates with
-EINVAL because of a early -EDEADLK, while if atomic_check ran to
completion it might have downgraded the modeset to a fastset.
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Testcase: kms_atomic
Link: https://patchwork.freedesktop.org/patch/msgid/20170815095706.23624-1-maarten.lankhorst@linux.intel.com
Fixes: d34f20d6 ("drm: Atomic modeset ioctl")
Cc: <stable@vger.kernel.org> # v4.0+
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>