drm/panfrost: Require the simple_ondemand governor

Panfrost depends on the simple_ondemand governor, and therefore
it's a required configuration. Select it.

Fixes: f3617b44

 ("drm/panfrost: Select devfreq")
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190605184859.9432-1-ezequiel@collabora.com