drm/i915: add i915_ioc32.h for compat

Keep reducing i915_drv.h.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200227170047.31089-1-jani.nikula@intel.com