cpufreq/amd-pstate-ut: Add test case for mode switches

There is a state machine in the amd-pstate driver utilized for
switches for all modes. To make sure that cleanup and setup works
properly for each mode add a unit test case that tries all
combinations.

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