i965/fs: Disable fs_inst assignment operator.

The fs_reg src array is going to turn into a pointer and we'd rather not
consider the implications of shallow copying fs_insts.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/drivers/dri/i965/brw_fs.h

@@ -185,6 +185,8 @@ public:
 };
 
 class fs_inst : public backend_instruction {
+   fs_inst &operator=(const fs_inst &);
+
 public:
    DECLARE_RALLOC_CXX_OPERATORS(fs_inst)