drm/amdgpu: fix scratch register access method in SRIOV

The scratch register should be accessed through MMIO instead of RLCG
in SRIOV, since it being used in RLCG register access function.

Fixes: d54762cc ("drm/amdgpu: nuke dynamic gfx scratch reg allocation")
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Gavin Wan <Gavin.Wan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>