drm/amdgpu: add support to configure MALL for sienna_cichlid (v2)

Enable Memory Access at Last Level (MALL) feature for sienna_cichlid.

v2: drop module option.  We need to add UAPI so userspace can
request MALL per buffer.

Reviewed-by: Christian König <christian.koenig@amd.com>
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>