drm/fb-helper: Fix missing kerneldoc include

This was lost in the code movement done in commit 8ab59da2
("drm/fb-helper: Move generic fbdev emulation into separate source
file").

Fixes: 8ab59da2

 ("drm/fb-helper: Move generic fbdev emulation into separate source file")
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20221107190928.1212116-1-daniel.vetter@ffwll.ch