Since

commit 890880dd
Author: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Date:   Fri Jan 4 09:56:10 2019 +0100

    drm: Auto-set allow_fb_modifiers when given modifiers at plane init

this is done automatically as part of plane init, if drivers set the
modifier list correctly. Which is the case here.
Acked-by: Maxime Ripard <maxime@cerno.tech>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Eric Anholt <eric@anholt.net>
Cc: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210413094904.3736372-11-daniel.vetter@ffwll.ch