DRM/Radeon: Fix TV DAC Load Detection for single CRTC chips.

The RN50 has a TV DAC but only a single CRTC. For load detection this
DAC is controlled by the primary CRTC.

Signed-off-by: Egbert Eich <eich@suse.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>