drm/i915: Add Plane color encoding support for YCBCR_BT2020

Currently the plane property doesn't have support for YCBCR_BT2020,
which enables the corresponding color conversion mode on plane CSC.
Enabling the plane property for the planes for GLK & ICL+ platforms.
Also as per spec, update the Plane Color CSC from YUV601_TO_RGB709
to YUV601_TO_RGB601.

V2: Enabling support for YCBCT_BT2020 for HDR planes on
    platforms GLK & ICL

V3: Refined the condition check to handle GLK & ICL+ HDR planes
    Also added BT2020 handling in glk_plane_color_ctl.

V4: Combine If-else into single If

V5: Drop the checking for HDR planes and enable YCBCR_BT2020
    for platforms GLK & ICL+.

V6: As per Spec, update PLANE_COLOR_CSC_MODE_YUV601_TO_RGB709
    to PLANE_COLOR_CSC_MODE_YUV601_TO_RGB601 as per Ville's
    feedback.

V7: Rebased

Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Kishore Kadiyala <kishore.kadiyala@intel.com>
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200601073544.11291-1-kishore.kadiyala@intel.com
