nir: Apply fragment color clamping to gl_FragData[] as well.

nir: Apply fragment color clamping to gl_FragData[] as well.

From the ARB_color_buffer_float spec:

   35. Should the clamping of fragment shader output gl_FragData[n]
       be controlled by the fragment color clamp.

       RESOLVED: Since the destination of the FragData is a color
       buffer, the fragment color clamp control should apply.

Fixes arb_color_buffer_float-mrt mixed on v3d.

Reviewed-by: Rob Clark <robdclark@gmail.com>