msm: kgsl: power state cleanup

Clean up kgsl_pwrctrl_sleep() and kgsl_pwrctrl_wake() to make
state transistions clearer.  Add kgsl_pwrctrl_request_state()
and kgsl_pwrctrl_set_state() to make it easier to debug the
state machine.

CRs-Fixed: 315833
Change-Id: I656ce8bd19feabd4186ef91dc031f8a6c6a7d09a
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>