drm/nouveau/tegra: Avoid pulsing reset twice

When the GPU powergate is controlled by a generic power domain provider,
the reset will automatically be asserted and deasserted as part of the
power-ungating procedure.

On some Jetson TX2 boards, doing an additional assert and deassert of
the GPU outside of the power-ungate procedure can cause the GPU to go
into a bad state where the memory interface can no longer access system
memory.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>