drm/radeon: add a module parameter to control deep color support

Some monitors seem to have problems with deep color enabled, even
though they claim to support it.  I'm not sure if the monitor
need a quirk or if the driver is doing something the monitor doesn't
like.  At this point lets just disable deep color by default like
we did for hdmi audio and work through the bugs so we can eventually
enable it by default.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=80531



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>