drm/amdgpu: fix use after free during BO move

The memory backing old_mem is already freed at that point, move the
check a bit more up.

Signed-off-by: Christian König <christian.koenig@amd.com>
Fixes: bfa3357e ("drm/ttm: allocate resource object instead of embedding it v2")
Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/1699


Acked-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org