drm/msm/a6xx: Fix stale rpmh votes from GPU

It was observed on sc7180 (A618 gpu) that GPU votes for GX rail and CNOC
BCM nodes were not removed after GPU suspend. This was because we
skipped sending 'prepare-slumber' request to gmu during suspend sequence
in some cases. So, make sure we always call prepare-slumber hfi during
suspend. Also, calling prepare-slumber without a prior oob-gpu handshake
messes up gmu firmware's internal state. So, do that when required.

Fixes: 4b565ca5 ("drm/msm: Add A6XX device support")
Cc: stable@vger.kernel.org
Signed-off-by: Akhil P Oommen <quic_akhilpo@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/639569/


Signed-off-by: Rob Clark <robdclark@chromium.org>