cpufreq: amd-pstate: change amd-pstate driver to be built-in type

Currently when the amd-pstate and acpi_cpufreq are both built into
kernel as module driver, amd-pstate will not be loaded by default
in this case.

Change amd-pstate driver as built-in type, it will resolve the loading
sequence problem to allow user to make amd-pstate driver as the default
cpufreq scaling driver.

Acked-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Gautham R. Shenoy <gautham.shenoy@amd.com>
Tested-by: Wyes Karny <wyes.karny@amd.com>
Signed-off-by: Perry Yuan <Perry.Yuan@amd.com>
Fixes: ec437d71

 ("cpufreq: amd-pstate: Introduce a new AMD P-State driver to support future processors")
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>