msm: vidc: Clean up the instance prior to destroying it

_destroy() is only present to reference manage the instance, and not
necessarily to house any logic.  Hence clean up the instance in
_close().

As a side-effect, it also fixes a bug where we send instance-related
commands to firmware after the instance was invalidated.  Hence if
firmware responds back, we ignore the reponses since we think that the
session is no longer valid.

Change-Id: Ia03af3ad9ef9251eddd03bf53ac46ff52bd051de
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>