drm/amd/display: Fix DSC not Enabled on Direct MST Sink

[WHY & HOW]
For the scenario when a dsc capable MST sink device is directly
connected, it needs to use max dsc compression as the link bw constraint.

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Reviewed-by: Roman Li <roman.li@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Fangzhi Zuo <jerry.zuo@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>