drm/amd/display: Avoid a NULL pointer dereference

commit 07b93a5 upstream.

[WHY]
Although unlikely drm_atomic_get_new_connector_state() or
drm_atomic_get_old_connector_state() can return NULL.

[HOW]
Check returns before dereference.

Cc: Mario Limonciello <[email protected]>
Cc: Alex Deucher <[email protected]>
Reviewed-by: Harry Wentland <[email protected]>
Signed-off-by: Mario Limonciello <[email protected]>
Signed-off-by: Alex Hung <[email protected]>
Tested-by: Dan Wheeler <[email protected]>
Signed-off-by: Alex Deucher <[email protected]>
(cherry picked from commit 1e5e8d672fec9f2ab352be121be971877bff2af9)
Cc: [email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: superm1

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

@@ -7761,6 +7761,9 @@ amdgpu_dm_connector_atomic_check(struct drm_connector *conn,
 	struct amdgpu_dm_connector *aconn = to_amdgpu_dm_connector(conn);
 	int ret;
 
+	if (WARN_ON(unlikely(!old_con_state || !new_con_state)))
+		return -EINVAL;
+
 	trace_amdgpu_dm_connector_atomic_check(new_con_state);
 
 	if (conn->connector_type == DRM_MODE_CONNECTOR_DisplayPort) {