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>

diff --cc drivers/gpu/drm/drm_crtc_helper.c
Simple merge

diff --cc drivers/gpu/drm/drm_irq.c
Simple merge

diff --cc drivers/gpu/drm/i915/intel_display.c
Simple merge

diff --cc include/drm/drmP.h
Simple merge