cpufreq: intel_pstate: Simplify intel_pstate_cpu_init()

The initial policy value set by intel_pstate_cpu_init() depends on
whether or not CONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE is set, but
that is not necessary, because the core will set the policy to
"performance" in cpufreq_init_policy() if the default governor is
"performance" anyway.

Accordingly, change intel_pstate_cpu_init() to always set policy
to CPUFREQ_POLICY_POWERSAVE initially to provide a valid fallback
value to cpufreq_init_policy() in case the default cpufreq governor
is neither "powersave" nor "performance".

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>