If we fail to pin the ggtt vma slot for the ppgtt page tables, we need
to unwind the locals before reporting the error. Or else on subsequent
attempts to bind the page tables into the ggtt, we will already believe
that the vma has been pinned and continue on blithely. If something else
should happen to be at that location, choas ensues.

Fixes: a2bbf714 ("drm/i915/gtt: Only keep gen6 page directories pinned while active")
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Cc: <stable@vger.kernel.org> # v4.19+
Reviewed-by: NMatthew Auld <matthew.william.auld@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181222030623.21710-1-chris@chris-wilson.co.uk
(cherry picked from commit d4de7535)
Signed-off-by: NJani Nikula <jani.nikula@intel.com>