msm: kgsl: Force the ME off before reading the microcode

On A3XX reading the microcode while the CP is running is dangerous
beyond words.  By setting the offset address for the read, we are
essentially changing the instruction pointer for the ME while it
is running causing opcode errors, gpu faults and/or system errors.

We do not want to stop the CP at the beginning of the snapshot
process beacuse doing so changes the RBBM status and the state of
the CP registers which may damage the debug effort.  But by the
time we get to the microcode read we no longer care about the state
of the registers so we can freely halt the ME just before starting
the read.

CRs-fixed: 585449
Change-Id: Ic0dedbad85d7ec26915f054dd597e226e8101a29
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>