drm/fb-helper: Initialize fb-helper's preferred BPP in prepare function

Initialize the fb-helper's preferred_bpp field early from within
drm_fb_helper_prepare(); instead of the later client hot-plugging
callback. This simplifies the generic fbdev setup function.

No real changes, but all drivers' fbdev code has to be adapted.

v3:
	* build with CONFIG_DRM_FBDEV_EMULATION unset (kernel test bot)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230125200415.14123-7-tzimmermann@suse.de