Allow to apply first brightness as -1

The initial brightness is -1 in DisplayDevice. It won't apply
the brightness(-1) for the first brightness. This allow an
caller to set the brightness to -1

Bug: 254657816
test: atest libsurfaceflinger_unittest:SetDisplayBrightnessTest
Change-Id: I3a87d0b505b8bf8ce411c86a061376aadba42a88
Merged-In: I3a87d0b505b8bf8ce411c86a061376aadba42a88