drm/amdgpu/nv: allow access to SDMA status registers

For access via ioctl for tools like umr and mesa.

Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Acked-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>