drm: Pass the display mode to drm_calc_timestamping_constants()

We don't really use hwmode anymore in i915, so eliminating its use
from the core code seems prudent. Just pass the appropriate mode
to drm_calc_timestamping_constants().

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