intel/fs: Fix constness of implied_mrf_writes() argument.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/intel/compiler/brw_fs.h

@@ -299,7 +299,7 @@ public:
 
    fs_reg interp_reg(int location, int channel);
 
-   int implied_mrf_writes(fs_inst *inst) const;
+   int implied_mrf_writes(const fs_inst *inst) const;
 
    virtual void dump_instructions();
    virtual void dump_instructions(const char *name);