Vblank timestamping improvements/fixes for Linux drm. [v2]

[Mario Kleiner]

Hi Dave,

this is v2 of the patch set for improving/restoring accuracy and
robustness of vblank timestamping and for fixing incompatibilities
with the PREEMPT_RT patches.

Could you please merge this for the next kernel? Would be good to have
the old accuracy restored as soon as possible. Thanks.

v2: Added the reviewed-by's of Ville and Alex, thanks for the review!
    Fixed multi-line code formatting as suggested by Ville.

Successfully tested on Intel and AMD Radeon hardware.

thanks,
-mario