drm/i915/gem: Hold the obj->vma.lock while walking the vma.list

Remember to take the lock before walking the obj->vma.list so that the
nodes do not change beneath us! E.g.,

i915_gem_object_bump_inactive_ggtt:387 GEM_BUG_ON(vma->vm != &i915->ggtt.vm)

Closes: https://gitlab.freedesktop.org/drm/intel/issues/691


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191204164527.3872783-1-chris@chris-wilson.co.uk