drm/amd/display: DSC passthrough is for DP-HDMI pcon (SST pcon)

if check over DSC passthrough is removed, as this is not for
MST use case. It is for DP-HDMI pcon use case. sst pcon is
detected as sst not mst. In sst pcon dsc passthrough message
will not get below log printed

Fixes: 9b035d08 ("drm/amd/display: Check & log if receiver supports MST, DSC & FEC.")
Suggested-by: Fangzhi Zuo <Jerry.Zuo@amd.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Fangzhi Zuo <Jerry.Zuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>