Use ring structure to access the cpu/gpu address of rptr/wptr.

v2: merge gfx10/sdma5/sdma5.2 patches

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