drm vblank regression fixes for Linux 4.4+

[Mario Kleiner]

Here is the series of patches with fixes for regressions in vblank
counting/timestamping caused by the rewrite of drm_update_vblank_count
in Linux 4.4. These are all meant for stable 4.4 and later.

I have tested them on radeon-kms and nouveau-kms by unplugging/replugging
displays, manual dpms off/on, dpms off/on due to screen blanking, system
suspend/resume, and mode setting to different resolutions and refresh rates,
checking the drm.debug logs to confirm that the large vblank counter jumps
no longer happen and the behavior of the vblank counter/ts around dpms and
modesetting is somewhat reasonable.

-mario