drm: Add missing static storage class specifier in drm_irq.c file

Fixes the following sparse warning:
drivers/gpu/drm/drm_irq.c:1239:6:
warning: symbol 'drm_handle_vblank_events' was not declared. Should it be static?
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_irq.c

@@ -1236,7 +1236,7 @@ int drm_wait_vblank(struct drm_device *dev, void *data,
 	return ret;
 }
 
-void drm_handle_vblank_events(struct drm_device *dev, int crtc)
+static void drm_handle_vblank_events(struct drm_device *dev, int crtc)
 {
 	struct drm_pending_vblank_event *e, *t;
 	struct timeval now;