gpu: nvgpu: vgpu: Get channel reference

- In vGPU code path, function vgpu_channel_abort_cleanup() does not
  obtain channel reference before using the channel structure
  (channel_gk20a)
- vgpu_channel_abort_cleanup() is called by vgpu_intr_thread() which
  runs commands obtained from interrupt queue from RM server.
- If there is a scenario where gk20a_channel_release() function runs
  before guest receives notification from RM server to abort channel
  cleanup, channel gets freed before vgpu_channel_abort_cleanup() runs.
- However, because vgpu_channel_abort_cleanup() does not take explicit
  reference to the channel, it ends up accessing structures
  (such as ch->g) which are set to NULL and thus we end up in a crash.
- This patch explicitly takes reference of channel before
  vgpu_channel_abort_cleanup() is called.
- If gk20a_channel_release() runs before vgpu_channel_abort_cleanup()
  and ends up freeing channel, we dont get reference to freed
  channel in vgpu_channel_abort_cleanup() and thus we return from
  function rather than continuing with freed channel as was the case
  previously.

Bug 200453473
JIRA EVLR-3411

Change-Id: I311043b2231336616b28246531cf8a0dc151b0cd
Signed-off-by: Deepak Bhosale <dbhosale@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/1932028
(cherry picked from commit b91228e506c14f04945e35b5c996f711bb30a155)
Reviewed-on: https://git-master.nvidia.com/r/1970807
Reviewed-by: Aparna Das <aparnad@nvidia.com>
Reviewed-by: Automatic_Commit_Validation_User
Tested-by: Karl Ding <kding@nvidia.com>
GVS: Gerrit_Virtual_Submit
Reviewed-by: Nirav Patel <nipatel@nvidia.com>
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>

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