drm/bridge: anx7625: Check GPIO description to avoid crash

As GPIO probe function "devm_gpiod_get_optional()" may return error
code, driver should identify GPIO desc as NULL to avoid crash.

Acked-by: Tzung-Bi Shih <tzungbi@google.com>
Signed-off-by: Xin Ji <xji@analogixsemi.com>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211119015804.3824027-1-xji@analogixsemi.com


Reviewed-by: Robert Foss <robert.foss@linaro.org>