zink: always unset vertex shader variant key data when changing last vertex stage
ensure that vertex key data is always zeroed when changing last stage since it will be updated before draw anyway and can only cause problems if left alone here
fixes the following caselist: dEQP-GLES31.functional.shaders.builtin_constants.tessellation_shader.max_tess_evaluation_texture_image_units dEQP-GLES31.functional.tessellation_geometry_interaction.feedback.tessellation_output_quads_geometry_output_points dEQP-GLES31.functional.ubo.random.all_per_block_buffers.25
cc: mesa-stable