etnaviv: keep references to pending resources

As long as a resource is pending in any context we must not destroy
it, otherwise we'll hit a classical use-after-free with fireworks.
To avoid this take a reference when the resource is first added to
the pending set and put the reference when no longer pending.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Jonathan Marek <jonathan@marek.ca>