drm/gma500: Remove never set dev_priv->rpm_enabled flag

The rpm_enabled flag is never set, remove it.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220909115646.99920-4-hdegoede@redhat.com