drm/i915: Check sink deep color capabilitis during HDMI .mode_valid()

Currently HDMI .mode_valid() only checks whether the source can do
deep color. Let's check whether the sink can do it as well.

Cc: Werner Sembach <wse@tuxedocomputers.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210511160532.21446-5-ville.syrjala@linux.intel.com


Reviewed-by: José Roberto de Souza <jose.souza@intel.com>