genxml: reduce amount of generated code

$ wc -l build/src/intel/genxml/genX_bits.h

250581 build/src/intel/genxml/genX_bits.h (before)
  5748 build/src/intel/genxml/genX_bits.h (after)

Signed-off-by: Lionel Landwerlin lionel.g.landwerlin@intel.com