drm: bridge: cdns-mhdp8546: Fix missing mutex unlock on error path

Add missing mutex unlock before returning from the error path in
cdns_mhdp_atomic_enable().

Fixes: 935a92a1 ("drm: bridge: cdns-mhdp8546: Fix possible null pointer dereference")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Qi Xi <xiqi2@huawei.com>
Reviewed-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20250904034447.665427-1-xiqi2@huawei.com


Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>