msm: kgsl: 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.

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