drm/i915/dp: Support DP ports YUV 4:2:0 output to GEN11

Bspec describes that GEN10 only supports capability of YUV 4:2:0 output to
HDMI port and GEN11 supports capability of YUV 4:2:0 output to both DP and
HDMI ports.

v2: Minor style fix.

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/20190521121721.32010-7-gwan-gyeong.mun@intel.com