drm/vkms: check plane_composer->map[0] before using it

Fix a copypasta error. The caller of compose_plane() already checks
primary_composer->map. In contrast, plane_composer->map is never
verified here before handling.

Fixes: 7938f421 ("dma-buf-map: Rename to iosys-map")
Reviewed-by: André Almeida <andrealmeid@riseup.net>
Signed-off-by: Tales Lelo da Aparecida <tales.aparecida@gmail.com>
Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220415111300.61013-2-tales.aparecida@gmail.com

drivers/gpu/drm/vkms/vkms_composer.c

@@ -157,7 +157,7 @@ static void compose_plane(struct vkms_composer *primary_composer,
 	void *vaddr;
 	void (*pixel_blend)(const u8 *p_src, u8 *p_dst);
 
-	if (WARN_ON(iosys_map_is_null(&primary_composer->map[0])))
+	if (WARN_ON(iosys_map_is_null(&plane_composer->map[0])))
 		return;
 
 	vaddr = plane_composer->map[0].vaddr;