turnip: make GS use correct varyings size from previous stage
Fixes:
dEQP-VK.tessellation.invariance.primitive_set.triangles_fractional_even_spacing_ccw
dEQP-VK.tessellation.invariance.outer_edge_division.triangles_fractional_even_spacing
dEQP-VK.tessellation.invariance.outer_edge_symmetry.triangles_fractional_odd_spacing_cw
dEQP-VK.tessellation.invariance.outer_edge_symmetry.quads_fractional_odd_spacing_ccw
dEQP-VK.tessellation.invariance.outer_edge_symmetry.isolines_equal_spacing_cw
dEQP-VK.tessellation.invariance.outer_edge_index_independence.triangles_equal_spacing_ccw
dEQP-VK.tessellation.invariance.outer_edge_index_independence.triangles_fractional_even_spacing_cw
dEQP-VK.tessellation.invariance.inner_triangle_set.triangles_equal_spacing
Other 4 failing tests in dEQP-VK.tessellation.invariance
have a different cause.