drm/amdgpu: fix dropped backing store handling in amdgpu_dma_buf_move_notify

bo->tbo.resource can now be NULL.

Signed-off-by: Christian König <christian.koenig@amd.com>
Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/1811


Acked-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211210083927.1754-1-christian.koenig@amd.com