soc/tegra: pmc: Remove additional check for a valid partition

The function tegra_powergate_is_powered() verifies that the partition
being queried is valid and so there is no need to check this before
calling tegra_powergate_is_powered() in powergate_show(). So remove this
extra check.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>