msm: camera: Validate camera instance before reserving buffer.

When VFE requests for a buffer, we check if a particular
instance is still streaming or not. Before checking this
condition, validate the instance itself since the instance
might have been closed by the application.

CRs-Fixed: 310426
Change-Id: Ifc8c2deaa5dcbbf4ea4e45bf03f8ade67b405b6e
Signed-off-by: Kiran Kumar H N <hurlisal@codeaurora.org>