drm/amdgpu: add lsdma block

Add Light SDMA (LSDMA) block and related function. LSDMA
is a small instance of SDMA mainly for kernel driver use.

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>