drm/i915/display: Let PCON convert from RGB to YCbCr if it can

If PCON has capability to convert RGB->YCbCr colorspace and also
to 444->420 downsampling then for any YUV420 only mode, we can
let the PCON do all the conversion. If the PCON supports
RGB->YCbCr conversion for all BT2020, BT709, BT601, choose
the one that is selected by userspace via connector colorspace
property, otherwise default to BT601.

v2: As suggested by Uma Shankar, considered case for colorspace
BT709 and BT2020, and default to BT601. Also appended dir
'display' in commit message.

v3: Fixed typo in condition for printing one of the error msg.

v4: As suggested by Uma Shankar:
-Fixed bug in determining the colorspace for RGB->YCbCr conversion.
-Fixed minor formatting issues
Also updated the commit message as per latest changes.

Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
[Jani: Fixed checkpatch PARENTHESIS_ALIGNMENT.]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201218103723.30844-16-ankit.k.nautiyal@intel.com