drm/i915/irq: add ilk_de_irq_postinstall()

Add a dedicated de postinstall function.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/f4a8570881e9be28a2406134b2898b8680f9e765.1691509966.git.jani.nikula@intel.com