staging: vboxvideo: Change address of scanout buffer on page-flip

Commit 2408898e3b ("staging: vboxvideo: Add page-flip support") only
calls vbox_crtc_do_set_base() on page-flips, but despite that function's
name it only pins the new fb, unpins the old fb and sets
vbox_crtc->fb_offset. It does not program the hardware to scan out at the
new vbox_crtc->fb_offset value.

This was causing only every other frame (assuming page-flipping between 2
buffers) to be shown since we kept scanning out of the old (now unpinned!)
buffer.

This commit fixes this by adding code to vbox_crtc_page_flip() to tell
the hardware to scanout from the new fb_offset.

Fixes: 2408898e3b ("staging: vboxvideo: Add page-flip support")
Cc: Steve Longerbeam <steve_longerbeam@mentor.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/vboxvideo/vbox_mode.c

@@ -323,6 +323,11 @@ static int vbox_crtc_page_flip(struct drm_crtc *crtc,
 	if (rc)
 		return rc;
 
+	mutex_lock(&vbox->hw_mutex);
+	vbox_set_view(crtc);
+	vbox_do_modeset(crtc, &crtc->mode);
+	mutex_unlock(&vbox->hw_mutex);
+
 	spin_lock_irqsave(&drm->event_lock, flags);
 
 	if (event)