drm/imx: only send commit done event when all state has been applied

Currently there is a small race window where we could manage to arm the
vblank event from atomic flush, but programming the hardware was too close
to the frame end, so the hardware will only apply the current state on the
next vblank. In this case we will send out the commit done event too early
causing userspace to reuse framebuffes that are still in use.

Instead of using the event arming mechnism, just remember the pending event
and send it from the vblank IRQ handler, once we are sure that all state
has been applied successfully.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
[p.zabel@pengutronix.de: inverted logic: done -> pending, added back
 spinlock in atomic_flush, commit message typo fix]
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>