msm: kgsl: Cleanup CP queue threshold programming logic

Simplify the logic for writing the CP queue threshold register
since readability is nine tenths of the law. More specifically,
don't do a billion (or 8) checks for the old targets.

Change-Id: Ia47088261602cbe455e00d5f8b1e74bbb967d3a1
Signed-off-by: Carter Cooper <ccooper@codeaurora.org>