drm/etnaviv: make it possible to allocate multiple events

This makes it possible to allocate multiple events under the event
spinlock. This change is needed to support 'sync'-points.

Changes v2 -> v3:
- wait for the completion of all events
- use 10sec timeout regardless of the number of events
- removed validation if there are enough free events
- fixed return value evaluation of event_alloc(..) in etnaviv_gpu_submit(..)

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