[i915] Prevent recursive batchbuffer flushing.

[i915] Prevent recursive batchbuffer flushing.

Workaround for recursive batchbuffer flushing: If the window is
moved, we can get into a case where we try to flush during a
flush.  What happens is that when we try to grab the lock for
the first flush, we detect that the window moved which then
causes another flush (from the intel_draw_buffer() call in
intelUpdatePageFlipping()).  To work around this we reset the
batchbuffer tail pointer before trying to get the lock.  This
prevent the nested buffer flush, but a better fix would be to
avoid that in the first place.