drm/radeon: Setup HDMI_CONTROL for hdmi deep color gcp's (v2)

Program HDMI_CONTROL to send general control packets
for hdmi deep color mode signalling at every video
frame if bpc > 8.

This is only supported on evergreen / DCE-4 and later.

v2: rebase

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>