drm/radeon/pm: autoswitch power state when in balanced mode

The current default of always using the performance power state leads
to increased power consumption of mobile devices, which have a dedicated
battery power state. Switch between the performance and battery power
state automatically, dpending on the current AC power status, when the
user asked for the balanced power state.

The user can still override this logic by asking for the performance
or battery power state explicitly.

Tested-by: Michel Dänzer <michel.daenzer@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>