radeonsi: fix multi plane buffers creation

When using 3 planes, the sequence produces this chain:
  plane0 -> plane2
This commit fixes this to produce:
  plane0 -> plane1 -> plane2

Fixes: 86e60bc2 ("radeonsi: remove si_vid_join_surfaces and use combined planar allocations")
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2193


Reviewed-by: Marek Olšák <marek.olsak@amd.com>
(cherry picked from commit e3e91ceb)