drm/amdgpu: drop scratch regs save and restore from S3/S4 handling

The expectation is that the base driver doesn't mess with these.
Some components interact with these directly so let the components
handle these directly.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>