mm-video: venc: Set default key-frame periodicity for VP8 codec

Setting a new default key-frame periodicity; from one I-frame for
every 15 P-frames to only one I-frame at the beginning of the stream.
Applications do not expect key frames unless explicitly requesting
them.

Bug: 12035976
Change-Id: I96681cb134a20cf89a76d517a4b7eef0f5e7a732
Signed-off-by: Arun Menon <avmenon@codeaurora.org>
Signed-off-by: Jorge Solano Altamirano <jsolano@codeaurora.org>
(cherry picked from commit f63397bd)