This will get rid of the following error:
[   74.730271] WARNING: CPU: 4 PID: 0 at drivers/gpu/drm/drm_vblank.c:614 drm_calc_vbltimestamp_from_scanoutpos+0x13e/0x2f0
[   74.730311] Modules linked in: vgem snd_hda_codec_hdmi snd_hda_codec_realtek snd_hda_codec_generic i915 x86_pkg_temp_thermal intel_powerclamp coretemp snd_hda_intel crct10dif_pclmul snd_hda_codec crc32_pclmul snd_hwdep broadcom ghash_clmulni_intel snd_hda_core bcm_phy_lib snd_pcm tg3 lpc_ich mei_me mei prime_numbers
[   74.730353] CPU: 4 PID: 0 Comm: swapper/4 Tainted: G     U           4.16.0-rc2-CI-CI_DRM_3822+ #1
[   74.730355] Hardware name: Dell Inc. XPS 8300  /0Y2MRG, BIOS A06 10/17/2011
[   74.730359] RIP: 0010:drm_calc_vbltimestamp_from_scanoutpos+0x13e/0x2f0
[   74.730361] RSP: 0018:ffff88022fb03d10 EFLAGS: 00010086
[   74.730365] RAX: ffffffffa0291d20 RBX: ffff88021a180000 RCX: 0000000000000001
[   74.730367] RDX: ffffffff820e7db8 RSI: 0000000000000001 RDI: ffffffff82068cea
[   74.730369] RBP: ffff88022fb03d70 R08: 0000000000000000 R09: ffffffff815d26d0
[   74.730371] R10: 0000000000000000 R11: ffffffffa0161ca0 R12: 0000000000000001
[   74.730373] R13: ffff880212448008 R14: ffff880212448330 R15: 0000000000000000
[   74.730376] FS:  0000000000000000(0000) GS:ffff88022fb00000(0000) knlGS:0000000000000000
[   74.730378] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   74.730380] CR2: 000055edcbec9000 CR3: 0000000002210001 CR4: 00000000000606e0
[   74.730382] Call Trace:
[   74.730385]  <IRQ>
[   74.730397]  drm_get_last_vbltimestamp+0x36/0x50
[   74.730401]  drm_update_vblank_count+0x64/0x240
[   74.730409]  drm_crtc_accurate_vblank_count+0x41/0x90
[   74.730453]  display_pipe_crc_irq_handler+0x176/0x220 [i915]
[   74.730497]  i9xx_pipe_crc_irq_handler+0xfe/0x150 [i915]
[   74.730537]  ironlake_irq_handler+0x618/0xa30 [i915]
[   74.730548]  __handle_irq_event_percpu+0x3c/0x340
[   74.730556]  handle_irq_event_percpu+0x1b/0x50
[   74.730561]  handle_irq_event+0x2f/0x50
[   74.730566]  handle_edge_irq+0xe4/0x1b0
[   74.730572]  handle_irq+0x11/0x20
[   74.730576]  do_IRQ+0x5e/0x120
[   74.730584]  common_interrupt+0x84/0x84
[   74.730586]  </IRQ>
[   74.730591] RIP: 0010:cpuidle_enter_state+0xaa/0x350
[   74.730593] RSP: 0018:ffffc9000008beb8 EFLAGS: 00000212 ORIG_RAX: ffffffffffffffde
[   74.730597] RAX: ffff880226b80040 RBX: 000000000031fc3e RCX: 0000000000000001
[   74.730599] RDX: 0000000000000000 RSI: ffffffff8210fb59 RDI: ffffffff820c02e7
[   74.730601] RBP: 0000000000000004 R08: 00000000000040af R09: 0000000000000018
[   74.730603] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000004
[   74.730606] R13: ffffe8ffffd00430 R14: 0000001166120bf4 R15: ffffffff82294460
[   74.730621]  ? cpuidle_enter_state+0xa6/0x350
[   74.730629]  do_idle+0x188/0x1d0
[   74.730636]  cpu_startup_entry+0x14/0x20
[   74.730641]  start_secondary+0x129/0x160
[   74.730646]  secondary_startup_64+0xa5/0xb0
[   74.730660] Code: e1 48 c7 c2 b8 7d 0e 82 be 01 00 00 00 48 c7 c7 ea 8c 06 82 e8 64 ec ff ff 48 8b 83 c8 07 00 00 48 83 78 28 00 0f 84 e2 fe ff ff <0f> 0b 45 31 ed e9 db fe ff ff 41 b8 d3 4d 62 10 89 c8 6a 03 41
[   74.730754] ---[ end trace 14b1345705b68565 ]---

Changes since v1:
- Don't try to apply CRC workaround when enabling pipe, it should already be enabled.
Changes since v2:
- Make crc functions for !DEBUGFS case inline.
- Pass intel_crtc to crc functions.
- Add comments to callsites.
Changes since v3:
- Cache selected source to pipe_crc->source.
- Set pipe_crc->skipped to MIN_INT during disable to close a race condition.
Changes since v4:
- Handle fallout from setting pipe_crc->source in irq handler.

Cc: Marta Löfstedt <marta.lofstedt@intel.com>
Reported-by: Marta Löfstedt <marta.lofstedt@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105185Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180308120202.52446-1-maarten.lankhorst@linux.intel.comReviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>