drm/panfrost: make devfreq optional again

Devfreq runtime usage was made mandatory, thus making panfrost fail to probe
on Amlogic S912 SoCs missing the "operating-points-v2" property.
Make it optional again, leaving PM_DEVFREQ selected by default.

Fixes: f3617b44

 ("drm/panfrost: Select devfreq")
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190605150233.32722-1-narmstrong@baylibre.com