gpu: implement PA_CL_UCP user clip planes through draw push constants

src/graphics/shader/shader.h

@@ -152,6 +152,8 @@ struct ShaderVertexInputInfo {
 	// LDS_SIZE of the GS stage a non-NGG vertex shader runs on; usually zero.
 	uint32_t                lds_size_dwords     = 0;
 	uint32_t                pa_cl_vs_out_cntl    = 0;
+	// PA_CL_CLIP_CNTL.UCP_ENA; set only on the last pre-rasterization stage.
+	uint32_t                user_clip_planes     = 0;
 	ShaderClipSpaceTransform clip_space;
 	ShaderMeshInputInfo      mesh;
 	ShaderTessellationInputInfo tess;