cpufreq/amd-pstate: convert mutex use to guard()

Using scoped guard declaration will unlock mutexes automatically.

Reviewed-by: Gautham R. Shenoy <gautham.shenoy@amd.com>
Link: https://lore.kernel.org/r/20241209185248.16301-5-mario.limonciello@amd.com


Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>