drm/armada: fix missing overlay wake-up

Nothing was waking up the overlay plane wait queue, so we were fully
reliant on the HZ/25 wait timing out to make progress.  Fix the lack
of wake-up.

We were also mis-handling the wait_event_timeout() return value - this
returns an unsigned integer of the remaining time, or zero on timeout
and the condition evaluated false.  Checking this for less than zero
is not sane.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>