cpufreq: amd-pstate: remove global header file

When extra warnings are enabled, gcc points out a global variable
definition in a header:

In file included from drivers/cpufreq/amd-pstate-ut.c:29:
include/linux/amd-pstate.h:123:27: error: 'amd_pstate_mode_string' defined but not used [-Werror=unused-const-variable=]
  123 | static const char * const amd_pstate_mode_string[] = {
      |                           ^~~~~~~~~~~~~~~~~~~~~~

This header is only included from two files in the same directory,
and one of them uses only a single definition from it, so clean it
up by moving most of the contents into the driver that uses them,
and making shared bits a local header file.

Fixes: 36c5014e ("cpufreq: amd-pstate: optimize driver working mode selection in amd_pstate_param()")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>