msm: vidc: add proper check for max secure instance allowed

Current driver design allows one additional secure instance
than max limit since the secure flags were updated later
for the current instance.
The change adds proper checks for the same.

Change-Id: If08e04bee09d91bb33604fea7debeda3da14850f
Signed-off-by: Dikshita Agarwal <dikshita@codeaurora.org>