msm: vidc: refine operating rate validation

- Use fps to calculate current workload when fps is
larger than operating rate.
- Fix a precision issue for using integer divide.
- Fix an unsigned integer overflow issue when max_freq
is smaller than total workload.
- Always allow decreasing operating rate.
- Consider work_route when calculating wordload.
- Support multi core case.

Change-Id: I6a431cd6b407cdae16a7dcd565a097a4a0f97fc2
Signed-off-by: Qiwei Liu <qiweil@codeaurora.org>