cpufreq: amd-pstate: Let user know amd-pstate is disabled

Commit 202e683d

 ("cpufreq: amd-pstate: add amd-pstate driver
parameter for mode selection") changed the driver to be disabled by
default, and this can surprise users.

Let users know what happened so they can decide what to do next.

Link: https://bugs.launchpad.net/bugs/2006942
Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Yuan Perry <Perry.Yuan@amd.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>