drm/i915: Clean up some comments in gmch irq code

Clean up some comments in the gmch irq code:
- drop redundant comments
- s/iir/IIR/ to make it clear it's referring to the register

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240927143545.8665-4-ville.syrjala@linux.intel.com


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