drm: omapdrm: Switch page flip to atomic helpers

The atomic page flip helper implements the page flip operation using
asynchronous commits.

As the legacy page flip was the last caller of omap_plane_mode_set(),
remove the function.

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