It's very confusing for userspace to have to deal with inconsistencies
here, and some drivers screwed this up a bit. Most just ommitted the
format list when they meant to say that only linear modifier is
allowed, but some also meant that only implied modifiers are
acceptable (because actually none of the planes registered supported
modifiers).

Now that this is all done consistently across all drivers, document
the rules and enforce it in the drm core.

v2:
- Make the capability a link (Simon)
- Note that all is lost before 5.1.

v3:
- Use drm_WARN_ON (Lyude)
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Acked-by: Maxime Ripard <maxime@cerno.tech>
Cc: Simon Ser <contact@emersion.fr>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Cc: Pekka Paalanen <pekka.paalanen@collabora.com>
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/20210506132343.2873699-1-daniel.vetter@ffwll.ch