Vblank timestamping enhancements for drm-next and 2.6.38rc

[Mario Kleiner]

Hi,

the following three patches are some small improvements and fixes to
the drm vblank timestamping. All fully tested on radeon rv530, r600 and
on i915 with 945gme chipset.

Please consider merging them for 2.6.38-rc as well.

Thanks,
-mario