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>