radv: Use correct vgpr_comp_cnt for VS if both prim_id and instance_id are needed.

Should take the max of the 2.

Fixes: ea337c8b "radv/gfx10: fix VS input VGPRs with the legacy path"
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
(cherry picked from commit 2e763f7c)