There's a bunch of atomic drivers who don't do this quite correctly,
luckily most of them aren't in wide use or people would have noticed
the tearing.

By making this the default we avoid the constant audit pain and can
additionally remove a ton of lines from vfuncs for a bit more clarity
in smaller drivers.

While at it complain if there's a cleanup_fb hook but no prepare_fb
hook, because that makes no sense. I haven't found any driver which
violates this, but better safe than sorry.

Subsequent patches will reap the benefits.

v2: It's neither ... nor, not not (Sam)
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
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/20210623162200.3372056-1-daniel.vetter@ffwll.ch