drm/amdgpu: Add helper function to get sdma index

Get the sdma index from ring

v2: refine function name

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Flora Cui <flora.cui@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>