gpu: nvgpu: simplify power management

We currenlty initialize both runtime PM and pm_domains frameworks
and use pm_domain to control runtime power management of NvGPU

But since GPU has a separate rail, using pm_domain is not
strictly required
Hence remove pm_domain support and use runtime PM only for all
the power management
This also simplifies the code a lot

Initialization in gk20a_pm_init()
- if railgate_delay is set, set autosuspend delay of runtime PM
- try enabling runtime PM
- if runtime PM is now enabled, keep GPU railgated
- if runtime PM is not enabled, keep GPU unrailgated
- if can_railgate = false, disable runtime PM and keep
  GPU unrailgated

Set gk20a_pm_ops with below callbacks for runtime PM
static const struct dev_pm_ops gk20a_pm_ops = {
.runtime_resume = gk20a_pm_runtime_resume,
.runtime_suspend = gk20a_pm_runtime_suspend,
.resume = gk20a_pm_resume,
.suspend = gk20a_pm_suspend,
}

Move gk20a_busy() to use runtime checks of pm_runtime_enabled()
instead of using compile time checks on CONFIG_PM

Clean up some pm_domain related code

Remove use of gk20a_pm_enable/disable_clk() since this
should be already done in platform specific unrailgate()/
railgate() APIs

Fix "railgate_delay" and "railgate_enable" sysfs to use
runtime PM calls

For VGPU, disable runtime PM during vgpu_pm_init()
With this, we will initialize vgpu with vgpu_pm_finalize_poweron()
upon first call to gk20a_busy()

Jira DNVGPU-57

Change-Id: I6013e33ae9bd28f35c25271af1239942a4fa0919
Signed-off-by: Deepak Nibade <dnibade@nvidia.com>
Reviewed-on: http://git-master/r/1163216
Reviewed-by: Automatic_Commit_Validation_User
GVS: Gerrit_Virtual_Submit
Reviewed-by: Terje Bergstrom <tbergstrom@nvidia.com>

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