msm: kgsl: Support speedbin specific throttle powerlevel

A powerlevel corresponds to a different GPU frequency in
different speedbins. Use speedbin specific throttle power level
instead of a global one for speedbin fused targets.

Change-Id: I6cea925957170ce64c4f0f8d13b40c5fad4214d4
Signed-off-by: Akhil P Oommen <akhilpo@codeaurora.org>