drm: renesas: shmobile: Move shmob_drm_crtc_finish_page_flip()

Move the shmob_drm_crtc_finish_page_flip() function up, to avoid having
to move it during the modification in the next change.

Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/4c87bb31403d142f8f146176cb859a10a28a3601.1694767209.git.geert+renesas@glider.be