drm/radeon/kms/pm: add mid profile

This adds an additional profile, mid, to the pm profile
code which takes the place of the old low profile.  The default
behavior remains the same, e.g., auto profile now selects between
mid and high profiles based on power source, however, you can now
manually force the low profile which was previously only available
as a dpms off state.  Enabling the low profile when the displays
are on has been known to cause display corruption in some cases.

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