intel_reg.rst was the first man page written in reStructuredText. Follow
suit with the rest of the man pages.

Add a generated defs.rst include file for definitions such as
intel-gpu-tools version. This replaces the MAN_SUBSTS sed script
substitutions from xorg-macros for old man pages.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>