x86/amd: Move amd_get_highest_perf() from amd.c to cppc.c

To prepare to let amd_get_highest_perf() detect preferred cores
it will require CPPC functions. Move amd_get_highest_perf() to
cppc.c to prepare for 'preferred core detection' rework.

No functional changes intended.

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