drm/radeon: remove overzealous warning in hdmi handling

hdmi audio works fine.  The warning just confuses users.

fixes:
https://bugzilla.kernel.org/show_bug.cgi?id=44341



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