msm: vidc: Fix a race condition in core instances list access

While checking the supported load, we are accessing core
instance list and there are chances that it can be
modified else where. Add a mutex lock while accessing
the core instances list to avoid such scenarios.

CRs-Fixed: 566451
Change-Id: I2ac5d1e51720557790a151af66e2e9fb5d3d152f
Signed-off-by: Rajeshwar Kurapaty <rkurapat@codeaurora.org>