drm: Move timestamping constants into drm_vblank_crtc

Collect the timestamping constants alongside the rest of the relevant
stuff under drm_vblank_crtc.

We can now get rid of the 'refcrtc' parameter to
drm_calc_vbltimestamp_from_scanoutpos().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>