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: I09dc527b1ff878604bf157e106bb3e5805a7e516
Signed-off-by: Tejas Prajapati <tpraja@codeaurora.org>