msm: vidc: Fix load calculation for specific core

Existing api to calculate core load for a give core,
returns the load corresponding to all the instances
in the core list. With the fix, the load is calculated
for the requested core.

CRs-Fixed: 2012520
Change-Id: Ic5a81a7d78ff110e9e8bc7870df5e03bb79c2c31
Signed-off-by: Vikash Garodia <vgarodia@codeaurora.org>