drm/i915: split irq hotplug function from display vtable

This provide a service from irq to display, so make it separate

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/05f533064709764dff8bcfef6a58f9a8482dc5bb.1632869550.git.jani.nikula@intel.com