drm/etnaviv: fix error code in event_alloc()

There are two "ret" variables declared in this function so setting
"ret = -EBUSY;" sets the wrong one.  The function ends up returning an
uninitialized variable.

Fixes: f098f9b8

 ("drm/etnaviv: move runtime PM handling to events")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>