drm/i915/irq: add dg1_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/cb6bb860fb7596d6b37c3e1e4c7657064d2d747a.1691509966.git.jani.nikula@intel.com