drm/i915: Hold mode_config lock whilst changing mode for lastclose()

drm/i915: Hold mode_config lock whilst changing mode for lastclose()

commit c291be9dba370ba696a0d482249a212cf5c15f45 upstream.

Upon lastclose(), we switch back to the fbcon configuration. This
requires taking the mode_config lock in order to serialise the change
with output probing elsewhere.

Reported-by: Oleksij Rempel <bug-track@fisher-privat.net>
References: https://bugs.freedesktop.org/show_bug.cgi?id=48652
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>