Introduce a boolean to know if opp table has been added.

With this, we can call panfrost_devfreq_fini() in case of error
and release what has been initialised.
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/20200710095409.407087-7-peron.clem@gmail.com