i965/vec4: dump subnr for FIXED_GRF
i965/vec4: dump subnr for FIXED_GRF

This came in handy when debugging the payload setup for Tess Eval,
since it prints correct subnr for attributes that can be loaded
in the second half of a register.

Reviewed-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler