msm: vidc: add support for dpb_buffer_count payload

[1] Send dpb buffer count payload to userspace during
    seq_change_event.
[2] Use min_buffer_req field in dpb_buffer_count pkt
    to calibrate driver internal buffer calculation.

Change-Id: If8cdee618282ea0c8d5affdaa4cddfe502e438f7
Signed-off-by: Govindaraj Rajagopal <grajagop@codeaurora.org>