pmdomain: core: Reset genpd->states to avoid freeing invalid data

If genpd_alloc_data() allocates data for the default power-states for the
genpd, let's make sure to also reset the pointer in the error path. This
makes sure a genpd provider driver doesn't end up trying to free the data
again, but using an invalid pointer.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Dhruva Gole <d-gole@ti.com>
Link: https://lore.kernel.org/r/20250402120613.1116711-1-ulf.hansson@linaro.org