drm/i915/irq: convert gen8_de_irq_handler() to void

The return value is not used for anything.

Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230515101738.2399816-1-jani.nikula@intel.com