radv: Fix gs_vgpr_comp_cnt for NGG VS without passthrough mode.

radv: Fix gs_vgpr_comp_cnt for NGG VS without passthrough mode.

When not in passthrough mode, the NGG shader needs to calculate the
primitive export value from the input primitive's vertex indices.

So, GS vertex offset 2 is needed when NGG has triangles
and isn't in passthrough mode.

Fixes: 7ad69e2f7ee10c0e7afc302b9324e7a320424dcb "radv: stop loading invocation ID for NGG vertex shaders"
Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15837>