drm/amdgpu: support fill mem for LSDMA

Support constant data filling in PIO mode for LSDMA.

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>