drm/amd/display: fix dereference of pointer aconnector when it is null

Currently pointer aconnector is being dereferenced by the call to
to_dm_connector_state before it is being null checked, this could
lead to a null pointer dereference.  Fix this by checking that
aconnector is null before dereferencing it.

Addresses-Coverity: ("Dereference before null check")
Reviewed-by: Mikita Lipski <mikita.lipski@amd.com>
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c

@@ -193,12 +193,11 @@ bool dm_helpers_dp_mst_write_payload_allocation_table(
 	 * that blocks before commit guaranteeing that the state
 	 * is not gonna be swapped while still in use in commit tail */
 
-	dm_conn_state = to_dm_connector_state(aconnector->base.state);
-
-
 	if (!aconnector || !aconnector->mst_port)
 		return false;
 
+	dm_conn_state = to_dm_connector_state(aconnector->base.state);
+
 	mst_mgr = &aconnector->mst_port->mst_mgr;
 
 	if (!mst_mgr->mst_state)