gpu: nvgpu: add support for powering off gpu

Add support for powering off IGPU for switching between
legacy to SMC mode/vice-versa or changing SMC configuration.
The power off can be issued as follows

echo 0 > /dev/nvgpu/igpu0/power

The following steps are done during a poweroff.
1) Deterministic channel idle
2) Acquire write_lock on l->busy semaphore.
3) Wait till power_usage decrements to indicate 0 active jobs.
4) Invoke pm_runtime_put_sync_suspend()
5) Invoke nvgpu_gr_remove_support() to clear existing GR memory.
6) Release write_lock on l->busy
7) Deterministic channel unidle.

Part of the sequence matches that of the gk20a_do_idle code.
The common parts are extracted into new functions
gk20a_block_new_jobs_and_idle() and gk20a_unblock_jobs()

For joint-rail case, the current implementation, does a railgate
and then sets pm_runtime_set_autosuspend_delay(-1) to disable
regular runtime resume/suspend.

Remove clearing of NVGPU_SUPPORT_MIG status during state change
ias it leads to inconsistencies.

Jira NVGPU-6920

Change-Id: I0b3eb3278176122ac061c1e8a94ebfb3c17c3925
Signed-off-by: Debarshi Dutta <ddutta@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/c/linux-nvgpu/+/2578501
Reviewed-by: svc_kernel_abi <svc_kernel_abi@nvidia.com>
Reviewed-by: Vijayakumar Subbu <vsubbu@nvidia.com>
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: Antony Clince Alex <aalex@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>
GVS: Gerrit_Virtual_Submit

drivers/gpu/nvgpu/os/linux/sysfs.c

@@ -306,6 +306,8 @@ static ssize_t railgate_enable_store(struct device *dev,
 		return -EINVAL;
 	}
 
+	nvgpu_log_info(g, "railgating is enabled %ld", railgate_enable);
+
 	if (railgate_enable) {
 		nvgpu_set_enabled(g, NVGPU_CAN_RAILGATE, true);
 		pm_runtime_set_autosuspend_delay(dev, g->railgate_delay);