drm/ttm: fix warning after moving resource to ghost obj

After we moved the resource to the ghost the bo->resource pointer needs
to be reset since the owner of the resource is now the ghost.

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/20210607175737.1405-1-christian.koenig@amd.com