When we walk the list of vma, or even for protecting against concurrent
framebuffer creation, we must hold the struct_mutex or else a second
thread can corrupt the list as we walk it.

Fixes regression from
commit d7f46fc4
Author: Ben Widawsky <benjamin.widawsky@intel.com>
Date:   Fri Dec 6 14:10:55 2013 -0800

    drm/i915: Make pin count per VMA

References: https://bugs.freedesktop.org/show_bug.cgi?id=89085Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Cc: stable@vger.kernel.org
Signed-off-by: NJani Nikula <jani.nikula@intel.com>