drm/lima: add timeout to drm scheduler init

After "5918045c drm/scheduler: rework job destruction", lima started
to leak memory due to buffers not being destroyed after job execution in
the drm scheduler.

This started happening because the drm scheduler only destroyed buffers
after cancelling the job timeout handler, and for lima this handler was
never started as lima specified a MAX_SCHEDULE_TIMEOUT timeout.

Lima seems to run well in its current state with a real timeout, so to
make it more aligned with the other drivers from now on, let's use a
real default timeout.

This also fixes the observed memory leaks.

The 500ms value was chosen as it is the current value for all other
embedded gpu drivers using drm sched.

Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Signed-off-by: Qiang Yu <yuq825@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190520224229.21111-1-nunes.erico@gmail.com