drm/i915: Hold struct_mutex for per-file stats in debugfs/i915_gem_object

drm/i915: Hold struct_mutex for per-file stats in debugfs/i915_gem_object

As we walk the obj->vma_list in per_file_stats(), we need to hold
struct_mutex to prevent alteration of that list.

Fixes: 1d2ac403ae3b ("drm: Protect dev->filelist with its own mutex")
Fixes: c84455b4bacc ("drm/i915: Move debug only per-request pid tracking from request to ctx")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101460
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170617115744.4452-1-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>