drm/amd/display: Align the returned error code with legacy DP

drm/amd/display: Align the returned error code with legacy DP

[Why]
For usb4 connector, AUX transaction is handled by dmub utilizing a differnt
code path comparing to legacy DP connector. If the usb4 DP connector is
disconnected, AUX access will report EBUSY and cause igt@kms_dp_aux_dev
fail.

[How]
Align the error code with the one reported by legacy DP as EIO.

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>