radv: don't set surf_index for stencil-only images

Fixes: f8d5b377c8b ('radv: set cb base tile swizzles for MRT speedups (v4)')
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108116
Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>