r600: Don't advertise support for scaled int16 vertex formats
They are not supported by the hardware. Fixes: dEQP-GLES2.functional.vertex_arrays.single_attribute.strides.buffer_0_32_short3_vec4_dynamic_draw_quads_1 dEQP-GLES2.functional.vertex_arrays.single_attribute.strides.buffer_0_32_short3_vec4_dynamic_draw_quads_256