drm/fb-helper: Add a FIXME that generic_setup is very confusing

drm/fb-helper: Add a FIXME that generic_setup is very confusing

I tried to fix this for real, but it's very sprawling and lots of
drivers get this mildly wrong one way or the other.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Acked-by: Simon Ser <contact@emersion.fr>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201211161113.3350061-2-daniel.vetter@ffwll.ch