drm/amdgpu: drop the backing store when DMA-buf imports are evicted

Instead of moving this to the SYSTEM domain just drop the backing store
and let the resulting allocation be freed.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Roger He <Hongbo.He@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>