intel/fs: Constify fs_inst::can_do_source_mods().

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>

src/intel/compiler/brw_fs.cpp

 }
 
 bool
-fs_inst::can_do_source_mods(const struct gen_device_info *devinfo)
+fs_inst::can_do_source_mods(const struct gen_device_info *devinfo) const
 {
    if (devinfo->gen == 6 && is_math())
       return false;

src/intel/compiler/brw_ir_fs.h

    bool is_copy_payload(const brw::simple_allocator &grf_alloc) const;
    unsigned components_read(unsigned i) const;
    unsigned size_read(int arg) const;
-   bool can_do_source_mods(const struct gen_device_info *devinfo);
+   bool can_do_source_mods(const struct gen_device_info *devinfo) const;
    bool can_do_cmod();
    bool can_change_types() const;
    bool has_source_and_destination_hazard() const;