DDC information can be accessed using AUX CH

Fixes failure to probe monitors on some systems with
DP bridge chips.

agd5f: minor fixes
Signed-off-by: NNiels Ole Salscheider <niels_ole@salscheider-online.de>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org