camera_v2: jpeg: Use buf_cmd.offset for v2p size calculation

Start using buf_cmd.offset when calculating the memory map
size for v2p. This is needed to enable ION input buffer support
for JPEG decoding.The input buffer has a header before the actual
bitstream.

Signed-off-by: Kamen Kirov <kkirov@codeaurora.org>
Change-Id: If9bdcefe3628bd21c9e76916e4d7e3dc3cc39447