drm/amd/display: Enable ASSR in DM

This patch enables ASSR if display declares such support in DPCD.

Signed-off-by: Stylon Wang <stylon.wang@amd.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>