drm: omapdrm: Rename omap_crtc_page_flip_locked to omap_crtc_page_flip

The operation is called page_flip, rename its implementation
accordingly.

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