drm/msm/a6xx: Add a helper for software-resetting the GPU

drm/msm/a6xx: Add a helper for software-resetting the GPU

Introduce a6xx_gpu_sw_reset() in preparation for adding GMU wrapper
GPUs and reuse it in a6xx_gmu_force_off().

This helper, contrary to the original usage in GMU code paths, adds
a readback+delay sequence to ensure that the reset is never deasserted
too quickly due to e.g. OoO execution going crazy.

Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/542758/
Signed-off-by: Rob Clark <robdclark@chromium.org>