never search for MCE strings in dmesg

'dmesg' element contains complete output of dmesg, therefore once kernel
log MCE all consecutive oopses has the MCE messages in 'dmesg' element.

'backtrace' element contains either oops's backtrace or the MCE message
whose scope is limited to a single oops.

Signed-off-by: Jakub Filak <jfilak@redhat.com>