drm/i915: make i915_suspend_switcheroo static

Looks like this was missed when unexporting, so let's keep sparse happy.

Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Fixes: 7f26cb88014a ("drm/i915: Unexport VGA switcheroo functions")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161202102411.19831-1-matthew.auld@intel.com

drivers/gpu/drm/i915/i915_drv.c

@@ -1522,7 +1522,7 @@ out:
 	return ret;
 }
 
-int i915_suspend_switcheroo(struct drm_device *dev, pm_message_t state)
+static int i915_suspend_switcheroo(struct drm_device *dev, pm_message_t state)
 {
 	int error;