drm: Add missing DP DSC extended capability definitions.

Adding DP DSC register definitions, we might need for further
DSC implementation, supporting MST and DP branch pass-through mode.

v2: - Fixed checkpatch comment warning
v3: - Removed function which is not yet used(Jani Nikula)

Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Acked-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221101094222.22091-2-stanislav.lisovskiy@intel.com