drm/etnaviv: fix flush sequence logic

The current logic uses the flush sequence from the current address
space. This is harmless when deducing the flush requirements for the
current submit, as either the incoming address space is the same one
as the currently active one or we switch context, in which case the
flush is unconditional.

However, this sequence is also stored as the current flush sequence
of the GPU. If we switch context the stored flush sequence will no
longer belong to the currently active address space. This incoherency
can then cause missed flushes, resulting in translation errors.

Fixes: 27b67278 ("drm/etnaviv: rework MMU handling")
Signed-off-by: Tomeu Vizoso <tomeu@tomeuvizoso.net>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Christian Gmeiner <cgmeiner@igalia.com>
Link: https://lore.kernel.org/r/20251021093723.3887980-1-l.stach@pengutronix.de