drm/amd/display: Disable DSC power gating in Diags

[why]
With DSC power gating enabled, one of the register reads times out occasionally,
causing a DSC test to fail.

[how]
Disable DSC power gating in Diags.
NOTE: This has to be reverted once the problems with DSC power gating are
resolved.

Signed-off-by: Nikola Cornij <nikola.cornij@amd.com>
Reviewed-by: Nevenko Stupar <Nevenko.Stupar@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>