i965/fs: Set compression only if writing two registers.
i965/fs: Set compression only if writing two registers.

We don't want to set compression control on a SIMD16 instruction
operating on words or smaller.

Reviewed-by: Jason Ekstrand <b186559eaedf698788888c8a2559b4efb4db10d6@intel.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler