drm/mcde: dsi: Fix return value check in mcde_dsi_bind()

The of_drm_find_bridge() function returns NULL on error, it doesn't return
error pointers so this check doesn't work.

Fixes: 5fc537bf

 ("drm/mcde: Add new driver for ST-Ericsson MCDE")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200430073145.52321-1-weiyongjun1@huawei.com