msm: kgsl: Move all platform drivers to the same module init function

There can only be one module_init() function per module.  Move all
three driver register calls into the same initialization function. The
ordering should still work correctly.

Change-Id: Ic0dedbadf7c69221a836ba3bbba362d0660f1f0f
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>