msm: venc: Modify vbvdelay logic

Modified vbvdelay logic with default behavior as
CBR+. Enable legacy CBR if resolution is < 720p.

Client can overwrite default behavior if
resolution is between VGA & 720p.

Change-Id: I9bd5ede782959780e5a5cb56eda0ae5e344e66aa
Signed-off-by: Darshana Patil <darshana@codeaurora.org>