kms_pipe_crc_basic occasionally hits "[drm:skl_set_cdclk] *ERROR* failed to inform PCU about cdclk change" on fi-skl-6700hq

My last attempt at fixing these wasn't entirely succesful perhaps:
commit 848496e5902833600f7992f4faa82dc1546051ba
Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
Date:   Wed Jul 13 16:32:03 2016 +0300

    drm/i915: Wait up to 3ms for the pcu to ack the cdclk change request on SKL

Also seen with kms_flip/basic-flip-vs-dpms on fi-skl-6770hq

Propability of this failure on fi-skl-6770hq is less than 10% (based on last 100 igt -t basic runs for CI_DRM:
CI_DRM_1646/fi-skl-6770hq/html/fi-skl-6770hq@CI_DRM_1646/igt@kms_pipe_crc_basic@read-crc-pipe-c
[  588.952203] [drm:skl_set_cdclk] *ERROR* failed to inform PCU about cdclk change
CI_DRM_1654/fi-skl-6770hq/html/fi-skl-6770hq@CI_DRM_1654/igt@kms_pipe_crc_basic@nonblocking-crc-pipe-a
[  519.129401] [drm:skl_set_cdclk] *ERROR* failed to inform PCU about cdclk change
CI_DRM_1655/fi-skl-6770hq/html/fi-skl-6770hq@CI_DRM_1655/igt@kms_pipe_crc_basic@read-crc-pipe-b
[  540.991991] [drm:skl_set_cdclk] *ERROR* failed to inform PCU about cdclk change
CI_DRM_1666/fi-skl-6770hq/html/fi-skl-6770hq@CI_DRM_1666/igt@pm_rpm@basic-pci-d3-state
[  609.849498] [drm:skl_set_cdclk] *ERROR* failed to inform PCU about cdclk change
CI_DRM_1693/fi-skl-6770hq/dmesg-during-piglit-run-1.log:
[  506.406787] [drm:skl_set_cdclk [i915]] *ERROR* failed to inform PCU about cdclk change

Note that with this sample every time it is different test case getting dmesg, and on CI_DRM_1693 none of the igt cases gets the dmesg-fail even though it does happen.

Priority decreased due to sporadic nature of the bug.

Haven't found any problem with the programming sequence and I couldn't reproduce the problem. A subsequent CDCLK change does succeed after the failed attempt, so how about increasing the timeout for now and investigate more if things still fail with that?

Reference to Imre's patchset to wait up to 3ms for the pcu to ack the cdclk change request on SKL: https://patchwork.freedesktop.org/series/15911/

Updated patchset:
https://patchwork.freedesktop.org/series/16375/

Fix merged to drm-tip.

Thanks Imre.

Closing as it is merged and passing for some time on all paltform in CI system