drm/plane: Fix typo in format_mod_supported documentation

Fix minor typo: "valdiate" -> "validate".

Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Reviewed-by: Simon Ser <contact@emersion.fr>
Signed-off-by: Simon Ser <contact@emersion.fr>
Link: https://patchwork.freedesktop.org/patch/msgid/20211226112503.31771-3-jose.exposito89@gmail.com