drm/amd/display: Add null pointer filter

[Why & How]
Add null pointer filter for logical integrity.
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Agustin Gutierrez <agustin.gutierrez@amd.com>
Signed-off-by: Jing Zhou <Jing.Zhou@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/gpio/gpio_service.c

@@ -647,7 +647,9 @@ enum gpio_result dal_ddc_set_config(
 void dal_ddc_close(
 	struct ddc *ddc)
 {
+	if (ddc != NULL) {
 		dal_gpio_close(ddc->pin_clock);
 		dal_gpio_close(ddc->pin_data);
+	}
 }