drm/radeon/kms/pm: add asic specific callbacks for getting power state (v2)

This also simplifies the code and enables reclocking with multiple heads
active by tracking whether the power states are single or multi-head
capable.

Eventually, we will want to select a power state based on external
factors (AC/DC state, user selection, etc.).

(v2) Update for evergreen

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