drm/i915: Use GEN3_IRQ_INIT() to init south interrupts in icp+

No reason not to use GEN3_IRQ_INIT() on icp+.

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201028213323.5423-19-ville.syrjala@linux.intel.com