drm/i915: Avoid compiler warning for maybe unused gu_misc_iir

/kisskb/src/drivers/gpu/drm/i915/i915_irq.c: warning: 'gu_misc_iir' may be used uninitialized in this function [-Wuninitialized]:  => 3120:10

Silence the compiler warning by ensuring that the local variable is
initialised and removing the guard that is confusing the older gcc.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Fixes: df0d28c1 ("drm/i915/icl: GSE interrupt moves from DE_MISC to GU_MISC")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180926104718.17462-1-chris@chris-wilson.co.uk