meta: memory leak of CopyPixels usage

Meta of CopyPixel generates a buffer object
but does not free it on cleanup.

Fixes: 37d11b13 (meta: Don't pollute the buffer object namespace in _mesa_meta_setup_vertex_objects)
Signed-off-by: Sergii Romantsov <sergii.romantsov@globallogic.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
(cherry picked from commit 7417b432)