msm: kgsl: restructure GPU power initialization

Move target specific initialization and setup into target
specific init functions. The change is required to port the GPU
driver to support future generation GPUs.

CRs-Fixed: 1053516
Change-Id: I808e247669fab61a6a64131858fe2f9e19754242
Signed-off-by: George Shen <sqiao@codeaurora.org>