drm/amdgpu: reorganize sdma v4 code to support more instances

This change is needed for Arcturus which has 8 sdma instances.
The CG/PG part is not covered for now.

Signed-off-by: Le Ma <le.ma@amd.com>
Acked-by: Snow Zhang &lt; <Snow.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>