drm/etnaviv: bump HW job limit to 4

The current limit of 2 leads to some GPU idle times, as the usual
IRQ latency leads to up to 3 jobs getting signaled at once with some
standard workloads.

A larger HW job limit might lead to slightly worse QoS, but we accept
that to not sacrifice GPU throughput in the common case.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>