drm/etnaviv: remove unnecessary local irq disable

The only event function that is called from IRQ context is event_free,
which is already using atomic bitmap operations, so we can avoid taking
the event spinlock in this function completely. As other the other
functions still using the event spinlock are all called from normal
process context, we can avoid disabling IRQs while holding the spinlock.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>