intel/fs/bank_conflicts: Don't touch Gen7 MRF hack registers.

Fixes: af2c320190 "intel/fs: Implement GRF bank conflict mitigation pass."
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104199
Reported-by: Darius Spitznagel <d.spitznagel@goodbytez.de>
Reviewed-by: Matt Turner <mattst88@gmail.com>

src/intel/compiler/brw_fs.h

@@ -277,7 +277,7 @@ public:
 
    struct brw_reg interp_reg(int location, int channel);
 
-   int implied_mrf_writes(fs_inst *inst);
+   int implied_mrf_writes(fs_inst *inst) const;
 
    virtual void dump_instructions();
    virtual void dump_instructions(const char *name);