cpufreq/amd-pstate: Modularize perf<->freq conversion

Delegate the perf<->frequency conversion to helper functions to reduce
code duplication, and improve readability.

Signed-off-by: Dhananjay Ugwekar <dhananjay.ugwekar@amd.com>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Gautham R. Shenoy <gautham.shenoy@amd.com>
Link: https://lore.kernel.org/r/20250205112523.201101-8-dhananjay.ugwekar@amd.com


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