drm vblank regression fixes for Linux 4.4+

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