Fix T76897: Lamp Power Not Updating from Driver

Lamps were not tagged with `ID_RECALC_SHADING` when they were updated
from drivers. As a result, Cycles considered the lamp as unchanged. This
is resolved by having a (seemingly non-functional) callback in a new
`LIGHT_UPDATE` depsgraph node.

This patch unconditionally adds the `LIGHT_UPDATE` node + the relation
from the lamp's PARAMETERS node.

Differential Revision: https://developer.blender.org/D7822

Reviewed by: brecht

source/blender/depsgraph/intern/node/deg_node_operation.h

@@ -170,6 +170,7 @@ enum class OperationCode {
   /* Shading. ------------------------------------------------------------- */
   SHADING,
   MATERIAL_UPDATE,
+  LIGHT_UPDATE,
   WORLD_UPDATE,
 
   /* Batch caches. -------------------------------------------------------- */