drm: add docs about the IN_FORMATS plane property

This is a standard property attached to planes in drm_universal_plane_init
when drm_mode_config.allow_fb_modifiers is true.

Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Daniel Stone <daniel@fooishbar.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/mHOtz80Wa-x3A5Lz5ETTM65VUMfW1j6b-iCuvIy1RzOM1EloHPqAb2MnbU8ygZi0ABaLrxuN6pzu0v3QRn83125C0bXuHoo-DQWWEd80fdw=@emersion.fr