drm/i915/irq: convert ibx_irq_reset() into ibx_display_irq_reset()

Observe that ibx_irq_reset() is really ibx_display_irq_reset(). Make it
so. Move to display, and call it directly from gen8_display_irq_reset()
instead of gen8_irq_reset().

Remove a nearby ancient stale comment while at it.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/20250409184702.3790548-1-jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>