drm/i915/dp: Add a config function for YCBCR420 outputs

This patch checks a support of YCBCR420 outputs on an encoder level.
If the input mode is YCBCR420-only mode then it prepares DP as an YCBCR420
output, else it continues with RGB output mode.
It set output_format to INTEL_OUTPUT_FORMAT_YCBCR420 in order to using
a pipe scaler as RGB to YCbCr 4:4:4.

v2:
  Addressed review comments from Ville.
  Style fixed with few naming.
  %s/config/crtc_state/
  %s/intel_crtc/crtc/
  If lscon is active, it makes not to call intel_dp_ycbcr420_config()
  to avoid to clobber of lspcon_ycbcr420_config() routine.
  And it move the 420_only check into the intel_dp_ycbcr420_config().

v3: Fix uninitialized return value and it is reported by Dan Carpenter.

v4:
  Addressed review comments from Ville.
  In order to avoid the extra indentation, it inverts if-clause on
  intel_dp_ycbcr420_config().
  Remove the error print where no errors print are allowed.

v6: Rebase

v7:
  Move intel_dp_get_colorimetry_status() to intel_dp from intel_psr.
  intel_dp_get_colorimetry_status() checks
  VSC_SDP_EXTENSION_FOR_COLORIMETRY_SUPPORTED bit in the
  DPRX_FEATURE_ENUMERATION_LIST register.
  And intel_dp_ycbcr420_config() uses intel_dp_get_colorimetry_status().

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190521121721.32010-2-gwan-gyeong.mun@intel.com