drm: omapdrm: Prevent processing the same event multiple times

The vblank interrupt is disabled after one occurrence, preventing the
atomic update event from being processed twice. However, this also
prevents the software frame counter from being updated correctly that
would require vblank interrupts to be kept enabled while the CRTC is
active.

In preparation for vblank interrupt fixes, make sure that the atomic
update event will be processed once only when the vblank interrupt will
be kept enabled.

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