camera: v4l2: Avoid duplicate put_pmem_file on single buffer.

For each video buffer, get_pmem_file is called when it's enqueued
the first time. When releasing the buffer, make sure put_pmem_file
is only done once.

CRs-fixed: 295111
Signed-off-by: Shuzhen Wang <shuzhenw@codeaurora.org>