genxml: Fix python crash when no dwords are found.

genxml: Fix python crash when no dwords are found.

If the 'dwords' dict is empty, max(dwords.keys()) throws an exception.
This case could happen when we have an instruction that is only an array
of other structs, with variable length.

v2:
   - Add another clause for empty dwords and make it work with python 3
   (Dylan)
   - Set the length to 0 if dwords is empty, and do not declare dw

Signed-off-by: Rafael Antognolli <rafael.antognolli@intel.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>