drm/rockchip: only call drm_fb_helper_hotplug_event if fb_helper present

Add a check for the presence of fb_helper to rockchip_drm_output_poll_changed()
to only call drm_fb_helper_hotplug_event if there is actually a fb_helper
available. Without this check I see NULL pointer dereferences when the
hdmi hotplug irq fires before the fb_helper got initialized.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>