drm/dp_helper: Add helpers to configure PCONs RGB-YCbCr Conversion

DP Specification for DP2.0 to HDMI2.1 Pcon specifies support for conversion
of colorspace from RGB to YCbCr.
https://groups.vesa.org/wg/DP/document/previewpdf/15651



This patch adds the relavant registers and helper functions to
get the capability and set the color conversion bits for rgb->ycbcr
conversion through PCON.

v2: As suggested in review comments:
-Fixed bug in the check condition in a drm_helper as reported by
 Dan Carpenter and Kernel test robot. (Dan Carepenter)
-Modified the color-conversion cap helper function, to accommodate
 BT709 and BT2020 colorspace. (Uma Shankar)
-Added spec details for the new cap for color conversion. (Uma Shankar)

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201218103723.30844-8-ankit.k.nautiyal@intel.com