drm/i915/gt: Fixup config ifdeffery for pm_suspend_target_state

pm_suspend_target_state is declared under CONFIG_PM_SLEEP but only
defined under CONFIG_SUSPEND. Play safe and only use the symbol if it is
both declared and defined.

Reported-by: kbuild-all@lists.01.org
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Fixes: a70a9e9 ("drm/i915: Defer rc6 shutdown to suspend_late")
Cc: Andi Shyti <andi.shyti@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191120182209.3967833-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_gt_pm.c

@@ -272,7 +272,7 @@ void intel_gt_suspend_prepare(struct intel_gt *gt)
 
 static suspend_state_t pm_suspend_target(void)
 {
-#if IS_ENABLED(CONFIG_PM_SLEEP)
+#if IS_ENABLED(CONFIG_SUSPEND) && IS_ENABLED(CONFIG_PM_SLEEP)
 	return pm_suspend_target_state;
 #else
 	return PM_SUSPEND_TO_IDLE;