drm: Use crtc_clock in drm_calc_timestamping_constants()

drm_calc_timestamping_constants() computes the pixel/line/frame
durations based on the crtc_ timing values. The corresponding pixel
clock is in mode->crtc_clock, so we need to use that instead of
mode->clock.

This should fix drm_calc_timestamping_constants() for frame packing
stereo modes.

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