drm/hdlcd: Fix up crtc_state->event handling

event_list just reimplemented what drm_crtc_arm_vblank_event does. And
we also need to send out drm events when shutting down a pipe.

With this it's possible to use the new nonblocking commit support in
the helpers.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Acked-by: Liviu Dudau <Liviu.Dudau@arm.com>