drm/i915/dp: Add support for "4:2:0 also" modes for DP

Currently we only support "4:2:0 also" modes on native HDMI.
Extend that support for DP as well.

With all the HDMI DFP TMDS clock handling sorted out this
is now going to work for both native DP and DP->HDMI
converters. As with native HDMI we first check if RGB
output is possible, and if not we try YCbCr 4:2:0 instead.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220322120015.28074-10-ville.syrjala@linux.intel.com
Reviewed-by: Uma Shankar <uma.shankar@intel.com>