drm/amdgpu: use LRU mapping policy for SDMA engines

Spreading the load across multiple SDMA engines can increase memory
transfer performance.

Signed-off-by: Andres Rodriguez <andresx7@gmail.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>