drm/amd/display: Enable DSC Flag in MST Mode Validation

[WHY & HOW]
When dsc is possible, MST mode validation includes:
1. if maximum dsc compression cannot fit into end to end bw, mode pruned
2. if native bw cannot fit into end to end bw, try to enabled dsc to see
   whether a feasible dsc config can be found
3. if native bw can fit into end to end bw, mode supported

Reviewed-by: Wayne Lin <wayne.lin@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>