msm: venc: Add BFrame support for encoder

BFrame can only be enabled/disabled based on a few other
settings like layers, LTR, codec etc. Add conditions for
enabling/disabling bframes and also adjust the GOP size
based on bframe decision.

CRs-Fixed: 2368458
Change-Id: I6ca193ed8e5d932d1de1dc3f4c4c6c683608790d
Signed-off-by: Vaibhav Deshu Venkatesh <vdeshuve@codeaurora.org>