drm/radeon/kms: mark 3D power states as performance

Fixes lack of power saving with multiple heads on
some desktop cards.

Fixes:
https://bugzilla.kernel.org/show_bug.cgi?id=16474



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