vidc:venc: Reject changing number of B-frames dynamically

The number of B-frames is not expected to be changed
at run time. The dynamic intra period settings are
allowed to change the number of P-frames and not B-frames.
This change will reject if number of B-frames is changed
in dynamic intra period setting.

CRs-fixed: 416057
(cherry picked from commit 94270bc0447074fc05c361f20fe58f47b9265c0e)

Change-Id: I09eddae28335f0eef6ee76639b55247304a12344