drm/i915/irq: add dedicated intel_display_irq_init()

Continue splitting display from the rest.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/45c247c9f5104f3e25bd8913644402a11ec3afaf.1691509966.git.jani.nikula@intel.com