drm/amdgpu: unbind in amdgpu_ttm_tt_unpopulate

Doing this in amdgpu_ttm_backend_destroy() is to late.

It turned out that this is not a good idea at all because it leaves pointers
to freed up system memory pages in the GART tables of the drivers.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210728130552.2074-2-christian.koenig@amd.com