drm: bridge: dw_hdmi: Enable GCP only for Deep Color

HDMI1.4b specification section 6.5.3:
Source shall only send GCPs with non-zero CD to sinks
that indicate support for Deep Color.

DW HDMI GCP default enabled, but only transmit CD
and do not handle AVMUTE, PP norDefault_Phase (yet).
Disable Auto GCP when 24-bit color for sinks that not support Deep Color.

Signed-off-by: Sandor Yu <Sandor.yu@nxp.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/78fa41e4fb3d3d53354034bc221fcf870dbac617.1649989179.git.Sandor.yu@nxp.com