drm/fh-helper: Split fbdev single-probe helper

Split the single-probe helper's implementation into multiple
functions and get locking and overallocation out of the way of
the surface setup. Simplifies later changes to the setup code.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230102112927.26565-11-tzimmermann@suse.de