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

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
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