intel: fix vblank crtc selection with DRI1 when only one pipe is enabled.

On Mobile chipsets, we often enable PipeB instead of PipeA, but the test
in here was insufficient, falling back to pipe A if the area
intersection returned zero. Therefore, in the case where a window went
off to the top of the left of the screen, it would freeze, waiting on
the wrong vblank.

Fix this mess by checking the sarea for a crtc being zero sized, and in
that case always default to the other one.

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