Set the drm_device to NULL, so that the newly created buffer object
doesn't appear to use the embedded gem object.

This is necessary, because otherwise no corresponding dma_resv_fini for
the dma_resv_init is called, resulting in a memory leak.

The dma_resv_fini in ttm_bo_release_list is only called if the embedded
gem object is not used, which is determined by checking if the
drm_device is NULL.

Bug: https://gitlab.freedesktop.org/drm/amd/issues/958
Fixes: 1e053b10 ("drm/ttm: use gem reservation object")
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Ahzo <Ahzo@tutanota.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/355089/