drm: Remove preempt_disable() from vblank timestamping code.

Preemption handling will get pushed into the kms
drivers in followup patches, to make timestamping
more robust and PREEMPT_RT friendly.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>