gpu: nvgpu: bail out from poweroff if channel suspend fails

gpu: nvgpu: bail out from poweroff if channel suspend fails

During gk20a_pm_prepare_poweroff(), if call to gk20a_channel_suspend()
fails, we proceed to disable other components and then return error.
But when genpd sees the error, it will abort the suspend sequence and
keep the device state as active.

But since we have already disabled all the components, GPU lands in
invalid state.

Hence, if channel_suspend() fails then do not proceed but return
the error immediately

Bug 200010416

Change-Id: I553a2a25832a1be4941bb6b6ce490c950cdbe7fa
Signed-off-by: Deepak Nibade <dnibade@nvidia.com>
Reviewed-on: http://git-master/r/422248
Reviewed-by: Terje Bergstrom <tbergstrom@nvidia.com>