drm/amd/display: Retrieve DSC Branch Decoder Caps

DSC extended branch decoder caps 0xA0 ~ 0xA2 is read from dsc_aux.
The dsc_aux is returned from drm dsc determination policy with the
right DSC capable MST branch device for decoding.

The values are all zero if DSC decoding at a MST BU with virtual DPCD;
The values are meaningful when DSC decoding at a MST BU without
virtual DPCD or at a DSC capable MST endpoint.

Signed-off-by: Fangzhi Zuo <Jerry.Zuo@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>