media: vicodec: reset last_src/dst_buf based on the IS_OUTPUT

When start_streaming was called both last_src_buf and last_dst_buf
pointers were set to NULL, but this depends on whether the capture
or output queue starts streaming.

When decoding with resolution changes in between the capture queue
has to restart streaming whenever a resolution change occurs. And
that would reset last_src_buf as well, which causes a problem if
the decoder was stopped by the application. Since last_src_buf
is now NULL, the LAST flag is never set for the last capture
buffer.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>