[media] s5p-mfc: Fixes for decode REQBUFS

- Honor return values from vb2_reqbufs on REQBUFS(0).

- Do not set the number of allocated buffers to 0 if userspace tries
  to request buffers again without freeing them.

- There is no need to verify correct instance state on reqbufs, as we will
  verify this in queue_setup().

- There is also no need to verify that vb2_reqbufs() was able to allocate enough
  buffers (pb_count) and call buf_init on that many buffers (i.e. dst_buf_count
  is at least pb_count), because this will be verified by second queue_setup()
  call as well and vb2_reqbufs() will fail otherwise.

- Only verify state is MFCINST_INIT when allocating, not when freeing.

- Refactor and simplify code.

Signed-off-by: Pawel Osciak <posciak@chromium.org>
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 <m.chehab@samsung.com>