drm/amd/display: Add a dc_state NULL check in dc_state_release

[How]
Check wheather state is NULL before releasing it.

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Allen Pan <allen.pan@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/core/dc_state.c

@@ -334,6 +334,7 @@ static void dc_state_free(struct kref *kref)
 
 void dc_state_release(struct dc_state *state)
 {
+	if (state != NULL)
 		kref_put(&state->refcount, dc_state_free);
 }
 /*