r600: Fix SSG when not all components are written

r600: Fix SSG when not all components are written

Make sure only those components are written to that are specified in the
write mask.

Fixes:
  dEQP-GLES2.functional.shaders.operator.common_functions.sign.lowp_float_vertex
  dEQP-GLES2.functional.shaders.operator.common_functions.sign.lowp_float_fragment
  dEQP-GLES2.functional.shaders.operator.common_functions.sign.mediump_float_vertex
  dEQP-GLES2.functional.shaders.operator.common_functions.sign.mediump_float_fragment
  dEQP-GLES2.functional.shaders.operator.common_functions.sign.highp_float_vertex
  dEQP-GLES2.functional.shaders.operator.common_functions.sign.highp_float_fragment
  dEQP-GLES2.functional.shaders.operator.common_functions.sign.lowp_vec3_vertex
  dEQP-GLES2.functional.shaders.operator.common_functions.sign.lowp_vec3_fragment
  dEQP-GLES2.functional.shaders.operator.common_functions.sign.mediump_vec3_vertex
  dEQP-GLES2.functional.shaders.operator.common_functions.sign.mediump_vec3_fragment
  dEQP-GLES2.functional.shaders.operator.common_functions.sign.highp_vec3_vertex
  dEQP-GLES2.functional.shaders.operator.common_functions.sign.highp_vec3_fragment
Signed-off-by: Gert Wollny <gw.fossdev@gmail.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>