drm/amdgpu: Verify DMA opearations from device are done

In case device remove is just simualted by sysfs then verify
device doesn't keep doing DMA to the released memory after
pci_remove is done.

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210512142648.666476-16-andrey.grodzovsky@amd.com