drm/panfrost: Use generic code for devfreq

Use dev_pm_opp_set_rate() instead of open coding the devfreq
integration, simplifying the code.

Reviewed-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191025134143.14324-2-steven.price@arm.com