drm/msm/a6xx: Ensure clean GMU state in a6xx_gmu_fw_start

drm/msm/a6xx: Ensure clean GMU state in a6xx_gmu_fw_start

While it's not very well understood, there is some sort of a fault
handler implemented in the GMU firmware which triggers when a certain
bit is set, resulting in the M3 core not booting up the way we expect
it to.

Write a magic value to a magic register to hopefully prevent that
from happening.

Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/543335/
Signed-off-by: Rob Clark <robdclark@chromium.org>