meta: Cleanup function for DrawTex

Buffer object was never freed, causing memory leaks.

Fixes: 76cfe2bc ("meta: Don't pollute the buffer object namespace in _mesa_meta_DrawTex")
CC: Ian Romanick <ian.d.romanick@intel.com>

Signed-off-by: Yevhenii Kolesnikov <yevhenii.kolesnikov@globallogic.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/1390>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/1390>
(cherry picked from commit b318bc20)