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>