drm: renesas: shmobile: Atomic conversion part 2

Implement atomic mode setting for the CRTC, using the existing dpms
callback.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/ca2a00200ef5ac899a6131087d0a30de1c806119.1694767209.git.geert+renesas@glider.be