drm/amd/display: implement DSC pass-through support

Currently, we only attempt to setup DSC at the virtual DPCD port,
however many modern displays come with DSC support and MST hubs can now
support DSC pass-through. So, to more optimally make use of the
available bandwidth, use DSC pass-through when possible by adding DSC
pass-through enablement support into the DSC enable sequence.

Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>