ARM: tegra: powergate: Don't error out if new state == old state

Don't treat it as an error if a partition is already in the same power
state when a user wants to power it on or off. This allows code to
proceed if no state change is required.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Stephen Warren <swarren@nvidia.com>