drm/i915/hpd: hide struct intel_hotplug_funcs

With intel_hpd_irq_setup() in i915_irq.c, struct intel_hotplug_funcs is
also only needed there.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/9a10dbca7ad298dde62b83f76e0df43a67bceeef.1643896905.git.jani.nikula@intel.com