drm/gem: Warn on trying to use a non-existing framebuffer plane

Warn if callers of drm_gem_fb_get_obj() try to use a GEM buffer for
a non-existing or unset plane.

Signed-off-by: Thomas Zimmermann <[e-mail]>
Reviewed-by: Javier Martinez Canillas <[e-mail]>
Tested-by: Noralf Trønnes <[e-mail]>
Acked-by: Christian König <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_gem_framebuffer_helper.c

···
 struct drm_gem_object *drm_gem_fb_get_obj(struct drm_framebuffer *fb,
 					  unsigned int plane)
 {
-	if (plane >= ARRAY_SIZE(fb->obj))
+	struct drm_device *dev = fb->dev;
+
+	if (drm_WARN_ON_ONCE(dev, plane >= ARRAY_SIZE(fb->obj)))
+		return NULL;
+	else if (drm_WARN_ON_ONCE(dev, !fb->obj[plane]))
 		return NULL;
 
 	return fb->obj[plane];