i915tex: Fix triple buffering after recent Mesa core changes.

i915tex: Fix triple buffering after recent Mesa core changes.

Remove superfluous _mesa_resize_framebuffer call which is now harmful because
it causes the third renderbuffer to have width/height 0, so Mesa refuses to
render to it.

In the long term, it would be nice to remove the hack in
intel_alloc_window_storage in favour of a proper Mesa interface for flipping
between more than two colour buffers.