drm/fb-helper: Remove preferred_bpp parameter from fbdev internals

Store the console's preferred BPP value in struct drm_fb_helper
and remove the respective function parameters from the internal
fbdev code.

The BPP value is only required as a fallback and will now always
be available in the fb-helper instance.

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/20230125200415.14123-6-tzimmermann@suse.de