gpu: nvgpu: Do not send WFI when finishing channel

gpu: nvgpu: Do not send WFI when finishing channel

The channel teardown process sends a WFI method to ensure that all
work has been completed. But we also preempt the channel a while
later, which also ensures that all work is completed.

Remove the code for submitting WFI, and rely on preemption to handle
idling the pipe.

Change-Id: I2af029184440ee73e70d377f15690ddaf9b8599f
Signed-off-by: Terje Bergstrom <tbergstrom@nvidia.com>
Reviewed-on: http://git-master/r/735067
(cherry picked from commit ac8b449bf22883dc1adf6da95ba715db398dbd72)
Reviewed-on: http://git-master/r/743861
Reviewed-by: Automatic_Commit_Validation_User
GVS: Gerrit_Virtual_Submit