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 <[email protected]>
Acked-by: Simon Ser <[email protected]>
Acked-by: Daniel Stone <[email protected]>
Acked-by: Pekka Paalanen <[email protected]>
Fixes: 455e00f1412f ("drm: Add getfb2 ioctl")
Cc: Daniel Stone <[email protected]>
Cc: Juston Li <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Ville Syrjälä <[email protected]>
Cc: Bas Nieuwenhuizen <[email protected]>
Cc: Marek Olšák <[email protected]>
Cc: "Wentland, Harry" <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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