Function declares a pointer return, so return one.

These two functions are just wrappers around display_create_*_surface
but weren't forwarding along the result of those calls as is implied by
their return value type.

This fixes a compile-time warning.

Signed-off-by: Bryce Harrington <bryce@canonical.com>

clients/window.c

@@ -453,9 +453,9 @@ display_create_surface(struct display *display,
 		       struct rectangle *rectangle)
 {
 #ifdef HAVE_CAIRO_GL
-	display_create_drm_surface(display, rectangle);
+	return display_create_drm_surface(display, rectangle);
 #else
-	display_create_shm_surface(display, rectangle);
+	return display_create_shm_surface(display, rectangle);
 #endif
 }
 
@@ -465,9 +465,9 @@ display_create_surface_from_file(struct display *display,
 				 struct rectangle *rectangle)
 {
 #ifdef HAVE_CAIRO_GL
-	display_create_drm_surface_from_file(display, filename, rectangle);
+	return display_create_drm_surface_from_file(display, filename, rectangle);
 #else
-	display_create_shm_surface_from_file(display, filename, rectangle);
+	return display_create_shm_surface_from_file(display, filename, rectangle);
 #endif
 }