glsl: Don't allow gl_FragData[i], with i>0 in GLES shaders

The OpenGL ES Shading Language specification describes the
values that may be output by a fragment shader. These are
gl_FragColor and gl_FragData[0]. Multiple render targets
are not supported in GLES.

Fixes dEQP test:
  * dEQP-GLES3.functional.shaders.fragdata.invalid_assign_to_1