drm/armada: handle atomic modeset crtc events

Prepare handling for atomic modeset CRTC events.  Currently, using the
transition helpers, CRTC events do not exist, but once we switch to
proper atomic modeset, they have to be handled.

We queue an event for the next vblank in two places:
- armada_drm_crtc_atomic_flush() provided we aren't doing an
  atomic modeset.
- armada_drm_crtc_commit() if we are committing a modeset.

This ensures that the event is sent at the correct time (after all
updates have been written to the hardware and after the following
vblank.)

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