msm: vidc: Update mbs per second calculation

When operating rate is less than fps, the behavior
is undefined. For better user experience use the
max value of fps and operating rate to scale the
clocks.

CRs-Fixed: 959695
Change-Id: I57aa3e8b4987cb1adf9fad514a1fd1e84d44ac3f
Signed-off-by: Vaibhav Deshu Venkatesh <vdeshuve@codeaurora.org>