drm: omapdrm: Handle events when enabling/disabling CRTCs

The driver currently handles vblank events only when updating planes on
an already enabled CRTC. The atomic update API however allows requesting
an event when enabling or disabling a CRTC. This currently leads to
event objects being leaked in the kernel and to events not being sent
out. Fix it.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>