[media] media: venus: vdec: add support for min buffers for capture

This adds support for V4L2_CID_MIN_BUFFERS_FOR_CAPTURE get control
in venus decoder, it is usable in case when the userspace wants
to know minimum capture buffers before calling request_buf for
capture queue in mem2mem drivers. Also this will fix an issue
found gstreamer v4l2videodec element, i.e. the video decoder
element cannot continue because the buffers are insufficient.

Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>