drm: Pass the display mode to drm_calc_vbltimestamp_from_scanoutpos()

Rather than using crtc->hwmode, just pass the relevant mode to
drm_calc_vbltimestamp_from_scanoutpos(). This removes the last hwmode
usage from core drm.

Reviewed-by:  <mario.kleiner.de@gmail.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>