drm/msm: Implement shutdown callback for adreno

Implement the shutdown callback for adreno gpu platform device
to safely shutdown it before a system reboot. This helps to avoid
futher transactions from gpu after the smmu is moved to bypass mode.

Signed-off-by: Akhil P Oommen <akhilpo@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>