msm: kgsl: Make a device specific scales_bandwidth function

To make it easier for the upcoming change to move GMU to a component
device make a real function to test if the target is capable of
scaling the bus bandwidth on the GMU.

Change-Id: Ic0dedbad255622428de2ac01a5833a9e28c9c3ac
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>