drm/etnaviv: reduce number of ktime_get calls in IRQ handler

A single IRQ might signal the completion of multiple jobs/fences
at once. There is no point in attaching a new timestamp to each
fence that only differs in when exactly the IRQ handler was able
to process this fence.

Get a single timestamp when the IRQ handler has determined that
there are completed jobs and reuse this for all fences that get
signalled by the handler.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Christian Gmeiner <cgmeiner@igalia.com>