msm: kgsl: Make a child device just for the bwmon governor

Now that we are creating the OPP tables at runtime, we don't need a
platform device defined in the device tree just for the bwmon governor.
Instead, create a simple child device and use that.

Change-Id: Ic0dedbadbebc2dac4f9a1e05045a7462742aaefd
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>