i965: Avoid flushing the batch for busy BOs for ARB_mbr with INVALIDATE_BUFFER.

i965: Avoid flushing the batch for busy BOs for ARB_mbr with INVALIDATE_BUFFER.

Unigine Tropics uses INVALIDATE_BUFFER and not UNSYNCHRONIZED to reset
the buffer object when its streaming wraps.  Don't penalize it by
flushing the batch at the wrap point, just allocate a new BO and get
to using it.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>