drm/radeon: Use correct value for unknown audio/video latency

Valid values are 1 to 251 for 0 to 500 ms latency, 0 for unknown
and 255 for audio/video unsupported by sink, according to HDMI 1.3 spec.
Also matches Radeon HDA verb 0xf7b documentation.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>