gpu: nvgpu: gpu railgate handling with runtime pm

Earlier implementation of railgate disable config is disabling
runtime pm during pm_init. This is causing multiple issues:
1. gpu rail will be on as soon as nvgpu driver probe is called.
   Actual gpu hw init may happen at much later point of time.
2. This is breaking railgate_enable sysfs node functionality.
   railgate_enable is not working if runtime pm is disabled.

To avoid all these issues for railgate disable, enable runtime pm
during pm_init and set auto-suspend delay to negative (-1), which
will disable runtime pm suspend calls.

Also fixed following issues along with this:
1. Updated railgate_enable debugfs implementation to use auto-suspend delay.
   To disable railgating:
   Set auto-suspend delay with negative value(-1) which will disable runtime
   pm suspend.
   To enable railgating:
   Set auto-suspend delay with railgate_delay value.
   Also removed redundant user_railgate_disabled gk20a device data and
   replaced with can_railgate, where ever it is applicable.
2. Initialized default railgate_delay to 500msec to avoid railgate
   on/off transitions with railigate enable from disabled state.
3. Created railgate_residency debug fs node irrespective of can_railgate
   initial state. This is helping with the case, where initial state of
   railgate state off and then railgate enable is done through sysfs node.

Bug 2073029

Change-Id: I531da6d93ba8907e806f65a1de2a447c1ec2665c
Signed-off-by: seshendra Gadagottu <sgadagottu@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/1694944
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>

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