drm/xe: make heci_gsc_irq_chip const

The irq_chip definition can be const, make it so.

Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240117122044.1544174-2-jani.nikula@intel.com


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