msm: vidc: Limit the max secure instances to 3 for sm6150

Limit the secure instances to 2 in platform data,
but as per the design driver allows 3rd instance
as well since the secure flags were updated later
for the current instance. Hence total no of
concurrent secure sessions support will be
max-secure-instances + 1.

CRs-Fixed: 2334065

Change-Id: I7a57e655fb7b40e5e2ad76936e3ccc7d90c9b4d8
Signed-off-by: Shrikara B <shrikara@codeaurora.org>