drm/amd/display: add dsc stream overhead for dp only

[why]
Based on hardware team recommendation this additional dsc overhead
is only required for DP DSC.

[how]
Add a check for is_dp and only apply the overhead if this flag is set.

Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Reviewed-by: Chris Park <Chris.Park@amd.com>
Acked-by: Wayne Lin <Wayne.Lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>