radv/gfx10: don't initialize VGT_INSTANCE_STEP_RATE_0

Only gfx9 and older use it to get InstanceID in VGPR1.
Ported from RadeonSI.

Cc: 19.2 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
(cherry picked from commit 0813c27d)