drm/radeon: add missing hdmi callbacks for rv6xx

When dpm was merged, I added a new asic struct for
rv6xx, but it never got properly updated when the
hdmi callbacks were added due to the two patch sets
being developed in parallel.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=69729

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