drm/nouveau: Use drm_fb_helper_output_poll_changed()

This driver can use drm_fb_helper_output_poll_changed() instead of
its own nouveau_fbcon_output_poll_changed().

Cc: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20171205182504.41923-8-noralf@tronnes.org

drivers/gpu/drm/nouveau/nouveau_vga.c

@@ -4,6 +4,7 @@
 
 #include <drm/drmP.h>
 #include <drm/drm_crtc_helper.h>
+#include <drm/drm_fb_helper.h>
 
 #include "nouveau_drv.h"
 #include "nouveau_acpi.h"
@@ -61,7 +62,7 @@ static void
 nouveau_switcheroo_reprobe(struct pci_dev *pdev)
 {
 	struct drm_device *dev = pci_get_drvdata(pdev);
-	nouveau_fbcon_output_poll_changed(dev);
+	drm_fb_helper_output_poll_changed(dev);
 }
 
 static bool