drm/etnaviv: fix dumping of active MMU context

gpu->mmu_context is the MMU context of the last job in the HW queue, which
isn't necessarily the same as the context from the bad job. Dump the MMU
context from the scheduler determined bad submit to make it work as intended.

Fixes: 17e4660a ("drm/etnaviv: implement per-process address spaces on MMUv2")
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>