drm/tegra: gr3d: Assert reset before power-gating

Tegra TRM documentation states that hardware should be in a default state
when power partition is turned off, i.e. reset should be asserted. This
patch adds the missing reset assertions.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>