drm/amd/display: Break out function to simply read aux reply

DRM's DP helpers take care of dealing with the error code for us. In
order not to step on each other's toes we'll need to be able to simply
read auch channel replies without further logic based on return values.

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Sun peng Li <Sunpeng.Li@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>