drm/radeon/atom: powergating fixes for DCE6

Power gating is per crtc pair, but the powergating registers
should be called individually.  The hw handles power up/down
properly.  The pair is powered up if either crtc in the pair
is powered up and the pair is not powered down until both
crtcs in the pair are powered down.  This simplifies
programming and should save additional power as the previous
code never actually power gated the crtc pair.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org