drm/amdgpu: drop scratch regs save and restore from GPU reset 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>