PM: devfreq: Fix simple_ondemand crashing on startup

simple_ondemands private data must be set to NULL, otherwise we would
run into a NULL pointer in kgsl_devfreq_get_dev_status().

Change-Id: I6cc6a8b11e3b58b8c3e3c26d43ee36949cf62351

Signed-off-by: flar2 <asegaert@gmail.com>