drm/msm/gpu: Wait for idle before suspending

System suspend uses pm_runtime_force_suspend(), which cheekily bypasses
the runpm reference counts.  This doesn't actually work so well when the
GPU is active.  So add a reasonable delay waiting for the GPU to become
idle.

Alternatively we could just return -EBUSY in this case, but that has the
disadvantage of causing system suspend to fail.

v2: s/ret/remaining [sboyd], and switch to using active_submits count
    to ensure we aren't racing with submit cleanup (and devfreq idle
    work getting scheduled, etc)
v3: fix inverted logic

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20220108180913.814448-2-robdclark@gmail.com
Signed-off-by: Rob Clark <robdclark@chromium.org>