drm/amd/display: read DP sink and DP branch hardware and firmware revision from DPCD

- define new dpcd address in drm
- implement new members in dpcd_caps to store values read from new dpcd address

Signed-off-by: Alvin lee <alvin.lee3@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>