drm/amd/display: Do not read DSC state if not in use

[why & how]
DSC may be power gated when coming out of S0i3, so avoid polling
DSC registers since it will fail anyways. Only read if it is known
that DSC is in use.

Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Signed-off-by: Ovidiu Bunea <Ovidiu.Bunea@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>