simplefb: formalize pseudo palette handling

Add a proper struct describing simplefb private data, with the palette in there,
instead of directly storing the palette in the fb_info->par pointer.

Signed-off-by: Luc Verhaegen <libv@skynet.be>
Acked-by: Stephen Warren <swarren@nvidia.com>
[hdegoede@redhat.com: drop unnecessary void * cast]
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Acked-by: Grant Likely <grant.likely@linaro.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>