drm/msm/a6xx: Handle GMU prepare-slumber hfi failure

When prepare-slumber hfi fails, we should follow a6xx_gmu_force_off()
sequence.

Signed-off-by: Akhil P Oommen <quic_akhilpo@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/498401/
Link: https://lore.kernel.org/r/20220819015030.v5.7.I54815c7c36b80d4725cd054e536365250454452f@changeid


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