drm/amd/display: rename dsc extended caps as dsc branch decoder caps

[why]
The capability fields are reserved for DSC branch
only to report the capability related to the
branch's DSC decoder.

Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>