drm/amd/display: Add one to EDID's audio channel count when passing to DC

DC takes channel count to mean the actual count. cea_sad's channels
represent it as number of channels - 1.

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org