msm: kgsl: Set busmon driver data

This links the busmon devfreq device to kgsl device
which is de-referenced in kgsl_busmon_target() in
order to propagate bus votes.

Change-Id: I1ac3f81e546e3fd6be6051a202e0d49db07978ee
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>