mm-video: vidc: venc: add set ctrl to request sequence header

The omx component can explicitly request for sequence header
using the set ctrl id - V4L2_CID_MPEG_VIDC_VIDEO_REQUEST_SEQ_HEADER.
The omx component should not rely on the driver to
send sequence header on it's own.

Change-Id: Ie5c2adc11c15b84a7f659d75357a09b7758ea199