drm/i915: Inline intel_dp_ycbcr420_config()

intel_dp_ycbcr420_config() is rather pointless. Just inline it
directly into intel_dp_compute_config(). This gets rid of the
ugly double assignment of output_format.

Not really sure what the best policy would be when the user
supplies a mode classified by the display as "YCbCr 4:2:0
only", but we know that we can't do YCbCr 4:2:0 output. For
now keep the current behaviour of just silently upgrade
it to RGB 4:4:4.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200924184156.24491-3-ville.syrjala@linux.intel.com


Reviewed-by: Uma Shankar <uma.shankar@intel.com>