drm/fbdev-generic: Initialize fb-helper structure in generic setup

Initialize the fb-helper structure immediately after its allocation
in drm_fbdev_generic_setup(). That will make it easier to fill it with
driver-specific values, such as the preferred BPP.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230125200415.14123-5-tzimmermann@suse.de