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
41bb0548b0