examples/power: fix for p-state

examples/power: fix for p-state

The vm_power_manager starts by setting the environment to acpi
using rte_power_set_env(PM_ENV_ACPI_CPUFREQ). This causes a problem
starting vm_power_manager when the system is using the intel_pstate
driver. The env should be set to none, or not called at all, because
the library now auto-detects the environment to be either acpi or
intel_pstate. This patch sets the environment to none so that the
library can successfully auto-detect.

Fixes: e6c6dc0f96c8 ("power: add p-state driver compatibility")

Signed-off-by: David Hunt <david.hunt@intel.com>