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.

CRs-Fixed: 2646173
Change-Id: I0540658b9c9487f6e3a4601a488a1add1e790dda
Signed-off-by: Tejas Prajapati <tpraja@codeaurora.org>