msm: kgsl: Make the gpubw governor immutable

The kgsl-busmon devfreq device is only set up to work with
the gpubw governor. So make it immutable to avoid unexpected
failures when re-assigning a different governor.

Change-Id: I545fa12efebf80aa38ff6270488f088fdaac2b84
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>