drm/i915: Include reminders about leaving no holes in uAPI enums

We don't want to pre-reserve any holes in our uAPI for that is a sign of
nefarious and hidden activity. Add a reminder about our uAPI
expectations to encourage good practice when adding new defines/enums.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190218094628.13522-1-chris@chris-wilson.co.uk