drm/msm: return the average load over the polling period

simple_ondemand interacts poorly with clamp_to_idle.  It only looks at
the load since the last get_dev_status call, while it should really look
at the load over polling_ms.  When clamp_to_idle true, it almost always
picks the lowest frequency on active because the gpu is idle between
msm_devfreq_idle/msm_devfreq_active.

This logic could potentially be moved into devfreq core.

Fixes: 7c0ffcd4

 ("drm/msm/gpu: Respect PM QoS constraints")
Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
Cc: Rob Clark <robdclark@chromium.org>
Link: https://lore.kernel.org/r/20220416003314.59211-3-olvaffe@gmail.com
Signed-off-by: Rob Clark <robdclark@chromium.org>