drm/etnaviv: fix failure path if model is zero

Fix the failure path to call pm_runtime_mark_last_busy() when failing
due to the model field being zero.

Acked-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>