drm/mcde: Fix uninitialized variable

We need to handle the case when of_drm_find_bridge() returns
NULL.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190618115245.13915-1-linus.walleij@linaro.org