Spatial dithering to 10 bpc depth was disabled for all DCE's.

Testing on DCE-8.3 and DCE-11.2 did not show any obvious ill
effects, but a measureable precision improvement (via colorimeter)
when displaying a fp16 framebuffer to a 10 bpc DP or HDMI connected
HDR-10 monitor.

v2: enable it for all DCEs (Alex)
Signed-off-by: NMario Kleiner <mario.kleiner.de@gmail.com>
Cc: Alex Deucher <alexdeucher@gmail.com>
Reviewed-by: Mario Kleiner <mario.kleiner.de@gmail.com> (v2)
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>