gpu: nvgpu: Add doxygen documentation in tsg.h

- Add doxygen documentation.
- Remove unused fields of nvgpu_tsg struct:
-- timeslice_timeout
-- timeslice_scale
- Remove unused functions:
-- nvgpu_tsg_set_runlist_interleave
- nvgpu_tsg_post_event_id is not supported in safety build.
  This function is moved under CONFIG_NVGPU_CHANNEL_TSG_CONTROL
  compiler flag.
- Below functions are moved under CONFIG_NVGPU_KERNEL_MODE_SUBMIT
  nvgpu_tsg_ctxsw_timeout_debug_dump_state
  nvgpu_tsg_set_ctxsw_timeout_accumulated_ms
- Rename
  gk20a_is_channel_active -> nvgpu_tsg_is_channel_active
  release_used_tsg -> nvgpu_tsg_release_used_tsg
- nvgpu_tsg_unbind_channel_common declared static
- Fix build issue when CONFIG_NVGPU_CHANNEL_TSG_CONTROL is disabled
  Remove CONFIG_NVGPU_CHANNEL_TSG_CONTROL for
  nvgpu_gr_setup_set_preemption_mode as it is needed in safety build.
  By default compute preemption mode will be set to WFI. CUDA will
  change it to CTA during context init time.

JIRA NVGPU-3595

Change-Id: I8ff6cabc8b892c691d951c37cdc0721e820a0297
Signed-off-by: Seema Khowala <seemaj@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/2151489
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>

drivers/gpu/nvgpu/os/posix/posix-tsg.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018, NVIDIA CORPORATION.  All rights reserved.
+ * Copyright (c) 2018-2019, NVIDIA CORPORATION.  All rights reserved.
  *
  * Permission is hereby granted, free of charge, to any person obtaining a
  * copy of this software and associated documentation files (the "Software"),
@@ -23,7 +23,9 @@
 #include <nvgpu/tsg.h>
 #include <nvgpu/gk20a.h>
 
+#ifdef CONFIG_NVGPU_CHANNEL_TSG_CONTROL
 void nvgpu_tsg_post_event_id(struct nvgpu_tsg *tsg,
 			     enum nvgpu_event_id_type event_id)
 {
 }
+#endif