drm/amd/display: fix can not turn on two displays due to DSC_RESOURCE failed.

[Why]
Can not turn on two displays at the same time with the asic having only one DSC.
DC_DSC_RESOURCE allocation failed.

[Solution]
Only add_dsc if the timing is dsc capable based on diag_dc and num_dsc

Signed-off-by: Charlene Liu <charlene.liu@amd.com>
Reviewed-by: Wesley Chalmers <Wesley.Chalmers@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>