[why]
First MST sideband message returns AUX_RET_ERROR_HPD_DISCON
on certain intel platform. Aux transaction considered failure
if HPD unexpected pulled low. The actual aux transaction success
in such case, hence do not return error.

[how]
Not returning error when AUX_RET_ERROR_HPD_DISCON detected
on the first sideband message.

v2: squash in fix (Alex)
Reviewed-by: NJerry Zuo <Jerry.Zuo@amd.com>
Acked-by: NBrian Chang <Brian.Chang@amd.com>
Signed-off-by: NTsung-hua Lin <Tsung-hua.Lin@amd.com>
Tested-by: NDaniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>