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: If04e459b1f27093d431a91dabd6b8ba3f4813067
Signed-off-by: Praneeth Paladugu <ppaladug@codeaurora.org>