msm: kgsl: Converts A6XX power counter value to microseconds

GPU DCVS needs power counter value in microseconds for
workload calculation. On A6XX, the power counter sourced
from XO clock instead of GFX3D clock. The conversion needs
adaptation.

CRs-Fixed: 2017390
Change-Id: I9ebaaa549d544322a419331d48a138e846795bd1
Signed-off-by: George Shen <sqiao@codeaurora.org>