msm: vidc: Set file private data to NULL on close

Set file private data to NULL on closing instance
to avoid invalid access after close.

Change-Id: I447791463d9dbe32e25237256589bb07be2c3a32
Signed-off-by: Surajit Podder <spodder@codeaurora.org>
Signed-off-by: Salman Syed <ssalman@codeaurora.org>