drm/amdgpu: support access regs outside of mmio bar

add indirect access support to registers outside of
mmio bar.

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>