cpufreq/amd-pstate: Cache EPP value and use that everywhere

Cache the value in cpudata->epp_cached, and use that for all callers.
As all callers use cached value merge amd_pstate_get_energy_pref_index()
into show_energy_performance_preference().

Check if the EPP value is changed before writing it to MSR or
shared memory region.

Reviewed-by: Gautham R. Shenoy <gautham.shenoy@amd.com>
Link: https://lore.kernel.org/r/20241209185248.16301-12-mario.limonciello@amd.com


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