drm/debugfs: fix potential NULL pointer dereference

encoder->funcs entry might be NULL, so check it first before calling its
methods. This fixes NULL pointer dereference observed on Rasberry Pi
3b/4b boards.

Fixes: caf525ed ("drm/encoder: register per-encoder debugfs dir")
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231205130631.3456986-1-m.szyprowski@samsung.com

drivers/gpu/drm/drm_debugfs.c

@@ -638,7 +638,7 @@ void drm_debugfs_encoder_add(struct drm_encoder *encoder)
 	debugfs_create_file("bridges", 0444, root, encoder,
 			    &bridges_fops);
 
-	if (encoder->funcs->debugfs_init)
+	if (encoder->funcs && encoder->funcs->debugfs_init)
 		encoder->funcs->debugfs_init(encoder, root);
 }