msm: camera: isp: variable should be accessed only if match is found

There is a possibility that the priority variable would
be accessed even for the HEAD node which would result in
out of bound errors, so access the elements of the
structure only if the handler is found.

Change-Id: I4bb96114d3947d466e5dd76b12054649e28b62fb
Signed-off-by: Tejas Prajapati <tpraja@codeaurora.org>