ACPI: CPPC: Add ACPI disabled check to acpi_cpc_valid()

Make acpi_cpc_valid() check if ACPI is disabled, so that its callers
don't need to check that separately.  This will also cause the AMD
pstate driver to refuse to load right away when ACPI is disabled.

Also update the warning message in amd_pstate_init() to mention the
ACPI disabled case for completeness.

Signed-off-by: Perry Yuan <Perry.Yuan@amd.com>
[ rjw: Subject edits, new changelog ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>