msm_vidc: venc: configure max-B-frames only if B is enabled

If B frames are initially enabled and disabled at the time
of configuration, max-B-frames will retain the non-zero value
set at the time of enablement. Explicitly reset max-B-frames
when B frames is set to zero, otherwise this setting may
interfere with other exclusive features (like temporal layers)

Change-Id: Iefa9b5b2becd0c0f4756fa81a396e3af0c45022a
Signed-off-by: Praveen Chavan <pchavan@codeaurora.org>