drm: mxsfb: Move vblank event arm to CRTC .atomic_flush()

The vblank event is armed in the plane .atomic_update(). This works fine
as we have a single plane, and was the only option when the driver was
using the drm_simple_kms_helper helper, but will break as soon as
multiple planes are supported. Move it to CRTC .atomic_flush().

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Stefan Agner <stefan@agner.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200727020654.8231-13-laurent.pinchart@ideasonboard.com