msm: kgsl: Remove "zero" power level

There was a time, long long ago, when having a "zero" power level made
sense because a "zero" powerlevel wasn't really "zero". That is no longer
the case and it is silly to force the DT to define it for EVERY. SINGLE.
TARGET. Get rid of it and assume that zero is, well... zero.

Change-Id: Ic0dedbadc5551585de893cdb5b3ed948a62aa430
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>