drm: use monotonic time in drm_calc_vbltimestamp_from_scanoutpos

For measuring duration we want to avoid that our start/end timestamps
jump, so use monotonic instead of real time for that.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: mario.kleiner
Signed-off-by: Dave Airlie <airlied@redhat.com>