fbdev: locking check for fb_set_suspend

Just drive-by, nothing systematic yet.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: "Michał Mirosław" <mirq-linux@rere.qmqm.pl>
Cc: Peter Rosin <peda@axentia.se>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Manfred Schlaegl <manfred.schlaegl@ginzinger.com>
Cc: Mikulas Patocka <mpatocka@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190528090304.9388-3-daniel.vetter@ffwll.ch