devfreq: Restrict gpu governors to gpu devfreq devices

These governors should fail if being set for any device
other than the gpu device. Do this by checking that the
devfreq device for which this governor is being set for
has the gpu device as it's parent device.

The vbif_bw governor immutable attribute is used
to restrict it being set for only gpu devfreq devices.
Because we don't intend to change the governor for
the devfreq devices which have this governor set as
default.

Change-Id: I391557c0d52a1f4ffba70e010401a0769587dea0
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>