drm: etnaviv: use previous GPU pipe state when pipe switching

Use the previous GPU pipe state when deciding which GPU caches should
be flushed prior to switching the current pipe.  This avoids infering
what the previously selected pipe was, and potentially flushing the
wrong caches.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>