drm/amdgpu: update the num of queue per pipe for mec on sienna_cichlid

The number of queue per pipe for mec on sienna_cichlid should be 4.

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>