drm/radeon/kms/atom: add support for crtc power gating

Each pair of crtcs (0/1, 2/3, 4/5) can be power gated.
Make sure the power is applied when the crtc is in use and
only power down the pair when both are off.

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