drm/etnaviv: reference MMU context when setting up hardware state

Move the refcount manipulation of the MMU context to the point where the
hardware state is programmed. At that point it is also known if a previous
MMU state is still there, or the state needs to be reprogrammed with a
potentially different context.

Cc: stable@vger.kernel.org # 5.4
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Tested-by: Michael Walle <michael@walle.cc>
Tested-by: Marek Vasut <marex@denx.de>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>