drm/radeon: Limit hdmi deep color bit depth to 12 bpc.

DCE-4/5/6 can't support more than 12 bpc deep color over hdmi,
so clamp to 12 bpc when a hdmi deep color capable display is
connected. This even makes sense on DCE-8+, which could do up
to 16 bpc, as driving with more than 12 bpc would only waste
video bandwidth as long as we don't support framebuffers with
more than 12 bpc depth.

On pre-DCE4 we clamp hdmi bit depth to 8 bpc, as those asics
don't support hdmi deep color.

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