msm: vidc: refine encoder workload calc

For HFR case, use operation rate instead of fps to
calculate the real workload for vsp.

Change-Id: Ib8b2331eabe83adcbad9f297a137b3a73b703e89
CRs-Fixed: 2241314
Signed-off-by: Qiwei Liu <qiweil@codeaurora.org>