drm/radeon/dpm: rework thermal state handling

1. Handle the the thermal state directly in the work handler.
Remove the state selection function since nothing else uses it now.
2. On some asics there is no thermal state, so we just use a regular
state and force the low performance state.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

diff --cc drivers/gpu/drm/radeon/radeon.h
Simple merge

diff --cc drivers/gpu/drm/radeon/radeon_pm.c
Simple merge