drm/amdgpu: fix amdgpu_fill_buffer (v2)

The mem start is relative to the domain in the address space, so this
worked only when VRAM was mapped at offset 0.

It also didn't handled multiple drm_mm_nodes for split VRAM.

v2: rebase on dma_fence renaming

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