drm/sysfb: simpledrm: Remove unused helper simpledrm_device_of_dev()

drm/sysfb: simpledrm: Remove unused helper simpledrm_device_of_dev()

After moving most of the mode-setting pipeline into drm_sysfb_helper.c,
simpledrm_device_of_dev() is no longer being used. Remove it.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reported-by: Jocelyn Falempe <jfalempe@redhat.com>
Closes: https://lore.kernel.org/dri-devel/20250407131344.139878-1-jfalempe@redhat.com/
Fixes: 177dfbdb7e67 ("drm/sysfb: Merge primary-plane functions")
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/20250407134753.985925-3-tzimmermann@suse.de