fbdev/offb: Allocate struct offb_par with framebuffer_alloc()

Move the palette array into struct offb_par and allocate both via
framebuffer_alloc(), as intended by fbdev. No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221219160516.23436-13-tzimmermann@suse.de