ir3: Preserve gl_ViewportIndex in the binning shader
Fixes dEQP-VK.draw.shader_viewport_index.* with TU_DEBUG=forcebin.
Fixes: efff7342 ("turnip: multiViewport and VK_EXT_shader_viewport_index_layer")
Fixes dEQP-VK.draw.shader_viewport_index.* with TU_DEBUG=forcebin.
Fixes: efff7342 ("turnip: multiViewport and VK_EXT_shader_viewport_index_layer")