drm/radeon: fix DAC interrupt handling on DCE5+

commit e9a321c6 upstream.

DCE5 and newer hardware only has 1 DAC.  Use the correct
offset.  This may fix display problems on certain board
configurations.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>