platform/x86/amd: pmc: Use guard(mutex)

Instead of using the `goto label; mutex_unlock()` pattern use
`guard(mutex)` which will release the mutex when it goes out of scope.

Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Link: https://lore.kernel.org/r/20241217194027.1189038-2-superm1@kernel.org


Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>