Vblank timestamping improvements/fixes for Linux drm.

[Mario Kleiner]

Hi all,

this patch set for the kernel pushes the latency sensitive bits of
vblank scanoutpos timestamping from the drm core into the kms drivers.

A change in the locking of the intel-kms driver for Linux 3.11 made
the old approach too inaccurate and also incompatible with the
PREEMPT_RT realtime kernel patch set. These patches fix that problem
and restore the old level of precision and reliability.

The patch set changes the prototype of driver->get_scanout_position()
to require/allow kms drivers to perform the ktime_get() system time
queries which go along with actual scanout position readout in a way
that provides maximum precision and to return those timestamps to
the drm. It also converts the only two kms drivers which use this api
so far (intel-kms and radeon-kms) to the new api and improves precision
and reliability of the intel-kms a lot.

Patches have been tested on Intel and AMD Radeon hardware and the Intel
bits have received some review and feedback by Ville Syrjälä.

Please review and apply if possible.

Thanks,
-mario