cpufreq/amd-pstate: Rework CPPC enabling

The CPPC enable register is configured as "write once".  That is
any future writes don't actually do anything.

Because of this, all the cleanup paths that currently exist for
CPPC disable are non-effective.

Rework CPPC enable to only enable after all the CAP registers have
been read to avoid enabling CPPC on CPUs with invalid _CPC or
unpopulated MSRs.

As the register is write once, remove all cleanup paths as well.

Reviewed-by: Dhananjay Ugwekar <dhananjay.ugwekar@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>