staging: drm/omap: send page-flip event after endwin

The endwin irq indicates that DSS has finished scanning out a buffer.
Use this to trigger page-flip event to userspace, so this happens
only *after* the previous buffer is finished.

Signed-off-by: Rob Clark <rob@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>