Setting layer brightness doesn't need nit info for readback tests

The nit information was used when we set exact nit values rather than a
relative brightness per layer. But we only need nit values for the
renderengine interface, which isn't tied to any hwc or panel
capabilities.

Bug: 301261125
Test: VtsHalGraphicsComposer3_TargetTest
Change-Id: I770dc5620648df2eab608e030c5e76cf190f315d