msm: kgsl: Fix for panic in z180 postmortem dumps

We were accessing z180 registers without holding
device mutex. This change ensures that we hold the
mutex before accessing z180's registers

CRs-Fixed: 644283
Change-Id: Ib6496c427fc6c9a3f97a6720adb80a390fe21b0d
Signed-off-by: Gaurav Gagrani <ggagrani@codeaurora.org>
Signed-off-by: Hareesh Gundu <hareeshg@codeaurora.org>