cpufreq: amd-pstate: Optimize amd_pstate_update_limits()

Don't take and release the mutex when prefcore isn't present and
avoid initialization of variables that will be initially set
in the function.

Reviewed-by: Gautham R. Shenoy <gautham.shenoy@amd.com>
Reviewed-by: Perry Yuan <perry.yuan@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>