drm/dp: Add helpers for DFP YCbCr 4:2:0 handling

Add helpers to determine whether the DFP supports
YCbCr 4:2:0 passthrough or YCbCr 4:4:4->4:2:0 conversion.

v2: Add kdocs (Lyude)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200904115354.25336-18-ville.syrjala@linux.intel.com


Reviewed-by: Lyude Paul <lyude@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>