drm/amd/display: Power-gate all DSCs at driver init time

[why]
DSC should be powered-on only on as-needed basis, i.e. if the mode
requires it

[how]
Loop over all the DSCs at driver init time and power-gate each

Signed-off-by: Nikola Cornij <nikola.cornij@amd.com>
Reviewed-by: Nevenko Stupar <Nevenko.Stupar@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>