drm/radeon: fix dig encoder selection on DCE61

commit 41fa54377057ab38bc3e08ebb46168a7daf2e63b upstream.

Was using the DCE41 code which was wrong. Fixes
blank displays on a number of Trinity systems.

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