[media] s5p-mfc: support MIN_BUFFERS query for encoder

Add V4L2_CID_MIN_BUFFERS_FOR_OUTPUT query for encoder.
Once mfc encoder state is HEAD_PARSED, which is sequence
header produced, dpb_count is avaialable. Let user space
query this value.

Signed-off-by: Kiran AVND <avnd.kiran@samsung.com>
Signed-off-by: Arun Kumar K <arun.kk@samsung.com>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>