msm: camera: Check for valid pcam before using it

When event notify is called, check if pcam is valid or not
to avoid NULL pointer deferences.

CRs-Fixed: 462215
Change-Id: I78590c06ff5132dce0a629e8496d11360201aafc
Signed-off-by: Raju P.L.S.S.S.N <rplsssn@codeaurora.org>