gpu: nvgpu: add railgate check in do_idle()

gpu: nvgpu: add railgate check in do_idle()

In gk20a_do_idle(), check gk20a rail status before returning.
If rail is off, then only return success otherwise return
failure

Bug 1376916
Bug 1487804

Change-Id: I6280bf06c686b8baa4d6f49e90f47148411c3e02
Signed-off-by: Deepak Nibade <dnibade@nvidia.com>
Reviewed-on: http://git-master/r/415281
(cherry picked from commit e30047a60e809aad55e396bec41b8662f4795505)
Reviewed-on: http://git-master/r/421698
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Terje Bergstrom <tbergstrom@nvidia.com>
GVS: Gerrit_Virtual_Submit