fbdev/atyfb: lock_fb_info can't fail

It's properly protected by reboot_lock.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Mikulas Patocka <mpatocka@redhat.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: "Ville Syrjälä" <syrjala@sci.fi>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190528090304.9388-16-daniel.vetter@ffwll.ch