iris: Implement INTEL_DEBUG=pc for pipe control logging.

iris: Implement INTEL_DEBUG=pc for pipe control logging.

This prints a log of every PIPE_CONTROL flush we emit, noting which bits
were set, and also the reason for the flush.  That way we can see which
are caused by hardware workarounds, render-to-texture, buffer updates,
and so on.  It should make it easier to determine whether we're doing
too many flushes and why.