drm/doc: Document that modifiers are always required for fb

Even for legacy userspace, since otherwise GETFB2 is broken and if you
switch between modifier-less and modifier-aware compositors, smooth
transitions break.

Also it's just best practice to make sure modifiers are invariant for
a given drm_fb, and that a modifier-aware kms drivers only has one
place to store them, ignoring any old implicit bo flags or whatever
else might float around.

Motivated by some irc discussion with Bas about amdgpu modifier
support.

Acked-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Acked-by: Simon Ser <contact@emersion.fr>
Acked-by: Daniel Stone <daniels@collabora.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Fixes: 455e00f1 ("drm: Add getfb2 ioctl")
Cc: Daniel Stone <daniels@collabora.com>
Cc: Juston Li <juston.li@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Cc: Marek Olšák <maraeo@gmail.com>
Cc: "Wentland, Harry" <harry.wentland@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200917164721.2038541-1-daniel.vetter@ffwll.ch