mesa: fix assertion in _mesa_drawbuffers()
Fixes failed assertion when _mesa_update_draw_buffers() was called
with GL_DRAW_BUFFER == GL_FRONT_AND_BACK. The piglit gl30basic hit
this.
Cc: "10.2" <mesa-stable@lists.freedesktop.org>
Reviewed-by:
Ian Romanick <ian.d.romanick@intel.com>