The code does not like to be interrupted when waiting for the first
vblank after opening a debugfs/crc channel, so don't.

[66285.716870] WARNING: CPU: 1 PID: 16615 at drivers/gpu/drm/drm_debugfs_crc.c:185 crtc_crc_open+0x1d0/0x1f0 [drm]
[66285.716877] Modules linked in: i915 intel_powerclamp crct10dif_pclmul crc32_pclmul crc32c_intel ghash_clmulni_intel cryptd intel_gtt i2c_algo_bit lpc_ich mfd_core drm_kms_helper syscopyarea sysfillrect sysimgblt fb_sys_fops prime_numbers drm video button autofs4 sd_mod ahci libahci libata i2c_i801 scsi_mod i2c_designware_platform i2c_designware_core i2c_core
[66285.716929] CPU: 1 PID: 16615 Comm: kms_frontbuffer Not tainted 4.11.0-rc5+ #7
[66285.716935] Hardware name: GIGABYTE GB-BXBT-1900/MZBAYAB-00, BIOS F8 03/02/2016
[66285.716941] Call Trace:
[66285.716955]  dump_stack+0x4d/0x6f
[66285.716966]  __warn+0xc1/0xe0
[66285.716975]  warn_slowpath_null+0x18/0x20
[66285.717004]  crtc_crc_open+0x1d0/0x1f0 [drm]
[66285.717014]  ? wake_atomic_t_function+0x50/0x50
[66285.717024]  full_proxy_open+0xf0/0x1b0
[66285.717032]  ? full_proxy_release+0x80/0x80
[66285.717042]  do_dentry_open.isra.17+0x14b/0x2d0
[66285.717051]  vfs_open+0x42/0x60
[66285.717064]  path_openat+0x5e7/0x13d0
[66285.717074]  ? refcount_dec_and_test+0x11/0x20
[66285.717081]  ? down_read+0xd/0x30
[66285.717087]  do_filp_open+0x85/0xf0
[66285.717093]  ? __vfs_write+0x23/0x120
[66285.717100]  ? __alloc_fd+0x3a/0x170
[66285.717107]  do_sys_open+0x11e/0x1f0
[66285.717113]  ? do_sys_open+0x11e/0x1f0
[66285.717119]  SyS_openat+0xf/0x20
[66285.717125]  entry_SYSCALL_64_fastpath+0x17/0x98
[66285.717131] RIP: 0033:0x7f5f2235146a
[66285.717135] RSP: 002b:00007ffd892e6bc0 EFLAGS: 00000246 ORIG_RAX: 0000000000000101
[66285.717142] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f5f2235146a
[66285.717147] RDX: 0000000000000000 RSI: 00007ffd892e6c40 RDI: 0000000000000006
[66285.717151] RBP: 00007ffd892e6b20 R08: 0000000000000000 R09: 000000000000000f
[66285.717156] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
[66285.717161] R13: 00007ffd892e6b10 R14: 0000000000000004 R15: 00000000007e61f4

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100610
Fixes: e8fa5671 ("drm: crc: Wait for a frame before returning from open()")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170407111712.13962-1-chris@chris-wilson.co.uk