drm/radeon/kms: add a power state type based on power state flags

The idea is to flag a power state with a certain type and use
that type to decide on what state to select.  On r6xx+, we
select a state and then transition between clock modes in that
state.  On pre-r6xx, we transition between states directly.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>