Improve max brightness handling

If we can detect the directory, why don't we detect maximum as well?

Additional patch by: Matt Mower
Replace clunky /nobrightness handling

Cleanup TWFunc::Set_Brightness() and blanktimer::getBrightness() a
bit, primarily for the purpose of relying on the value in
tw_has_brightnesss_file instead of checking for a phony brightness
path.

Change-Id: Ib22595df53cefa8db7a1172a581984c42ad461c6