drm/imx: only send event on crtc disable if kept disabled

The event will be sent as part of the vblank enable during the modeset
if the crtc is not being kept disabled.

Fixes: 5f2f9115 ("drm/imx: atomic phase 3 step 1: Use atomic configuration")

Signed-off-by: Robert Beckett <bob.beckett@collabora.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>