drm/i915/skl: gen9 uses the same bind_vma() vfuncs as gen6+

Temporary plug a BUG() while waiting for a better solution. See:

  http://lists.freedesktop.org/archives/intel-gfx/2014-January/038132.html



However Chris was looking at cleaning-up this as well, so went for the
easy intermediate solution instead.

Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>