drm/amd/pm: drop unnecessary gfxoff controls

Those gfxoff controls added for some specific ASICs are unnecessary.
The functionalities are not affected without them. Also to align with
other ASICs, they should also be dropped.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>