drm/i915: Only enable DFP 4:4:4->4:2:0 conversion when outputting YCbCr 4:4:4

drm/i915: Only enable DFP 4:4:4->4:2:0 conversion when outputting YCbCr 4:4:4

Let's not enable the 4:4:4->4:2:0 conversion bit in the DFP unless we're
actually outputting YCbCr 4:4:4. It would appear some protocol
converters blindy consult this bit even when the source is outputting
RGB, resulting in a visual mess.

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/2914
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210111164111.13302-1-ville.syrjala@linux.intel.com
Fixes: 0fb1fe383348 ("drm/i915: Do YCbCr 444->420 conversion via DP protocol converters")
Reviewed-by: Jani Nikula <jani.nikula@intel.com>