msm: kgsl: Set bus scaling function during gmu probe

When setting the bus scaling function, we are checking for gmu
based targets before gmu has probed. Simplify this by setting the
bus scaling function to interconnect by default. Then, replace
it with gmu based routine during gmu probe. This works because
only gmu based targets set bus votes via gmu.

Change-Id: Ib332e736b39e0f38b1028049422ba95bd0370839
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>