v3dv: assign driver locations on fragment shader output variables

We were getting driver location 0 on all output variables, which meant
that our color writes were always being redirected to render target 0.

Fixes dEQP-VK.pipeline.framebuffer_attachment.diff_* which uses multiple
render targets.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6766>
1 file changed