drm/tegra: dc: Do not needlessly deassert reset

Commit 9c012700 ("drm/tegra: dc: Add powergate support") changed the
driver's ->probe() implementation to deassert the module reset, and with
there being nobody else to assert it until ->remove() there is no need
to deassert again later on.

Signed-off-by: Thierry Reding <treding@nvidia.com>