drm/i915: clear the stolen fb before resuming

Similar to
commit 88afe715
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Sun Dec 16 12:15:41 2012 +0000

    drm/i915: Clear the stolen fb before enabling

but on the resume path.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=57191


Reported-and-tested-by: Nikolay Amiantov <nikoamia@gmail.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@vger.kernel.org (3.9 only)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>