msm: kgsl: Calculate busy interval considering GPU freq as float

GPU freq could be non-integral value and treating it as an integer
to calculate GPU busy insterval leads to higher busy interval
and more often we see negative idle stats. This change caculates
busy interval by treating gpu freq to the correct precision.

Change-Id: Ic54130803d7a0436b4ba31b09915eb7165d16590
CRs-fixed: 454972
Signed-off-by: Vinay Roy <vroy@codeaurora.org>