radv: Don't use vgpr indexing for outputs on GFX9.

Due to LLVM bugs. Fixes a bunch of dEQP-VK.glsl.indexing.*
tests.

Fixes: e38685cc 'Revert "radv: disable support for VEGA for now."'
Reviewed-by: Dave Airlie <airlied@redhat.com>
(cherry picked from commit 6ce55045)
[Bas Nieuwenhuizen: resolve conflicts]

Conflicts:
        src/amd/vulkan/radv_shader.c