drm/amd/display: Ignore First MST Sideband Message Return Error

[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.

Cc: stable@vger.kernel.org # 4.18+
Signed-off-by: Fangzhi Zuo <Jerry.Zuo@amd.com>
Acked-by: Solomon Chiu <solomon.chiu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>