[why]
Need to support 6 bpp for 420 pixel encoding only.

[how]
Add a dc function to determine what bpp range can be supported
for given pixel encoding.
Signed-off-by: Wenjing Liu <Wenjing.Liu@amd.com>
Reviewed-by: Nikola Cornij <Nikola.Cornij@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>