drm/msm/devfreq: Restrict idle clamping to a618 for now

Until we better understand the stability issues caused by frequent
frequency changes, lets limit them to a618.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Tested-by: John Stultz <john.stultz@linaro.org>
Tested-by: Caleb Connolly <caleb.connolly@linaro.org>
Link: https://lore.kernel.org/r/20211018153627.2787882-1-robdclark@gmail.com
Signed-off-by: Rob Clark <robdclark@chromium.org>