drm/vmwgfx: Fix overlay when using Screen Targets

This code was never updated to support Screen Targets.
Fixes a bug where Xv playback displays a green screen instead of actual
video contents when 3D acceleration is disabled in the guest.

Fixes: c8261a961ece ("vmwgfx: Major KMS refactoring / cleanup in preparation of screen targets")
Reported-by: Doug Brown <[e-mail]>
Closes: https://lore.kernel.org/all/[e-mail]
Signed-off-by: Ian Forbes <[e-mail]>
Tested-by: Doug Brown <[e-mail]>
Signed-off-by: Zack Rusin <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/vmwgfx/vmwgfx_overlay.c

···
 {
 	struct vmw_escape_video_flush *flush;
 	size_t fifo_size;
-	bool have_so = (dev_priv->active_display_unit == vmw_du_screen_object);
+	bool have_so = (dev_priv->active_display_unit != vmw_du_legacy);
 	int i, num_items;
 	SVGAGuestPtr ptr;