fbdev: sh_mobile_hdmi: framebuffer notifiers have to be registered

A previous patch added a framebuffer notifier to sh_mobile_hdmi.c,
but did not register it with the framebuffer core. This patch adds
such a registration and moves the notifier block into dynamically
allocated per-device private data.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>