drm/i915/irq: split out hotplug irq handling

Split hotplug irq handling out of i915_irq.[ch] into
display/intel_hotplug_irq.[ch].

The line between the new intel_hotplug_irq.[ch] and the existing
intel_hotplug.[ch] needs further clarification, but the first step is to
move the stuff out of i915_irq.[ch].

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230515101738.2399816-2-jani.nikula@intel.com