msm: camera: isp: Reset last flush req id as part of release

KMD maintains a list of fixed contexts that are re-circulated,
need to reset last flush req id at the time of release to avoid
any stale entries.

Change-Id: Id5a28511dea1826cab01bf3dcc39216ddcaaf6a9
Signed-off-by: Karthik Anantha Ram <kartanan@codeaurora.org>
Signed-off-by: Harsh Shah <harshs@codeaurora.org>