drm/bridge: lt8912b: fix incorrect handling of of_* return values

A static analysis shows several issues in the driver code at
probing time.

DT parsing errors were bad handled and could lead to bugs:
  - Bad error detection;
  - Bad release of resources

Fixes: 30e2ae94

 ("drm/bridge: Introduce LT8912B DSI to HDMI bridge")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Suggested-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Adrien Grassein <adrien.grassein@gmail.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210415183639.1487-1-rdunlap@infradead.org
Signed-off-by: Robert Foss <robert.foss@linaro.org>