drm/i915: Call drm_calc_timestamping_constants() earlier

Update the pixel/line/frame duration information when we switch to the
new pipe config. This will keep the timestamping constants in better
sync with the real hardware state.

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