msm: kgsl: Move RGMU to the a6xx target specific space

Much like the GMU code, the GMU code belongs in the a6xx target specific
space. Luckily, it is a lot easier to move. Move everything to
adreno_a6xx_rgmu.[ch] and rename the functions and structs.

Change-Id: Ic0dedbadf9f16f458f1cb1e540960a074464b348
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>