msm: vidc: Fix unknown control initialization

Private controls should have ID's starting from MPEG
controls. Fix unknown control ID based on this.

CRs-Fixed: 2368458
Change-Id: Ibdc18e51eb887ace01de7744a43921bfb10a1edb
Signed-off-by: Vaibhav Deshu Venkatesh <vdeshuve@codeaurora.org>