gpu: nvgpu: fix memory leak of completed_waiter

gpu: nvgpu: fix memory leak of completed_waiter

- completed_waiter is required only if register_irq flag is set
- when this flag is not set, we still allocate memory for waiter
  which results into memory leak

- hence allocate memory for waiter only if register_irq flag is set

Bug 200007643
Bug 200007297

Change-Id: I373a1d13b42254d3d72590ef2c36f59b72d35501
Signed-off-by: Deepak Nibade <dnibade@nvidia.com>
Reviewed-on: http://git-master/r/414894
(cherry picked from commit 15a85801a26740315887b5118d84e358b7fdcde8)
Reviewed-on: http://git-master/r/414893
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Terje Bergstrom <tbergstrom@nvidia.com>