drm/panfrost: don't use pfdevfreq.busy_count to know if hw is idle

This use devfreq variable that will be lock with spinlock in future
patches. We should either introduce a function to access this one
but as devfreq is optional let's just remove it.

Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Signed-off-by: Clément Péron <peron.clem@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200710095409.407087-4-peron.clem@gmail.com