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>
diff --git a/drivers/gpu/drm/radeon/radeon.h b/drivers/gpu/drm/radeon/radeon.h
index 4b0bbf8..a2e1ed8 100644
--- a/drivers/gpu/drm/radeon/radeon.h
+++ b/drivers/gpu/drm/radeon/radeon.h
@@ -102,6 +102,7 @@
 extern int radeon_hard_reset;
 extern int radeon_vm_size;
 extern int radeon_vm_block_size;
+extern int radeon_deep_color;
 
 /*
  * Copy from radeon_drv.h so we don't have to include both and have conflicting