drm/i915/rps: fix stale reference to i915->irq_lock

drm/i915/rps: fix stale reference to i915->irq_lock

The RPS code has been switched from using i915->irq_lock to gt->irq_lock
years ago in commit d762043f7ab1 ("drm/i915: Extract GT powermanagement
interrupt handling"). Fix the stale comment referencing i915->irq_lock,
which has also ceased to exist.

Reported-by: Gustavo Sousa <gustavo.sousa@intel.com>
Closes: https://lore.kernel.org/r/174656703321.1401.8627403371256302933@intel.com
Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://lore.kernel.org/r/20250507083136.1987023-1-jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>