drm/fb-helper: unexport drm_fb_helper_generic_probe

Not sure we don't yet have this as a patch somewhere ...

Motivation is that the automatic lifetime management of the generic fbdev
code is quite tricky, and it'll get even more tricky. Allowing drivers
to just use the fb_probe looks like a recipe for disaster.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Noralf Trønnes <noralf@tronnes.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191112175048.1581-1-daniel.vetter@ffwll.ch