drm/amdgpu: add basics for SDMA page queue support

Just the common helper and a new ring in the SDMA instance.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>