drm/i915/skl: Add missing SKL ID

Used by production device:
    Intel(R) Iris(TM) Graphics P555

Cc: <stable@vger.kernel.org>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170227112256.20060-1-michal.winiarski@intel.com