msm: kgsl: Finish the GMU transition to a6xx

Finish the transition of the GMU code to a6xx. Rename the gmu_device to
a6xx_gmu_device and move the header files. All GMU code should now exist
entirely in the a6xx domain.

Change-Id: Ic0dedbad3640906599421373bef38217fafbf6dc
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>