drm/amd/display: Guard against ddc_pin being NULL for AUX

[Why]
In the case where we don't support DMUB aux but we have DPIA links
in the configuration we might try to message AUX using the legacy
path - where DDC pin is NULL. This causes a NULL pointer dereference.

[How]
Guard against NULL DDC pin, return a failure for aux engine acquire.
Reviewed-by: Michael Strauss <Michael.Strauss@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@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/dce/dce_aux.c

@@ -572,6 +572,11 @@ int dce_aux_transfer_raw(struct ddc_service *ddc,
 
 	memset(&aux_req, 0, sizeof(aux_req));
 
+	if (ddc_pin == NULL) {
+		*operation_result = AUX_RET_ERROR_ENGINE_ACQUIRE;
+		return -1;
+	}
+
 	aux_engine = ddc->ctx->dc->res_pool->engines[ddc_pin->pin_data->en];
 	if (!acquire(aux_engine, ddc_pin)) {
 		*operation_result = AUX_RET_ERROR_ENGINE_ACQUIRE;