msm: kgsl: Replace jiffy sampling with usec

Jiffies only allow timescales on the order of 10msec.  Switch
to using kernel time functions for better granularity.  Pwrscale
samples should be submitted ideally every 10-20msec.

Change-Id: If9e5bed33b7d990818e0337af1af6d7d0b95bd11
Signed-off-by: Lucille Sylvester <lsylvest@codeaurora.org>