[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: NWenjing Liu <Wenjing.Liu@amd.com>
Reviewed-by: NNikola Cornij <Nikola.Cornij@amd.com>
Acked-by: NLeo Li <sunpeng.li@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>