drm/amd/display: Update HPO I/O When Handling Link Retrain Automation Request

[WHY]
Previous multi-display HPO fix moved where HPO I/O enable/disable is performed.
The codepath now taken to enable/disable HPO I/O is not used for compliance
test automation, meaning that if a compliance box being driven at a DP1 rate
requests retrain at UHBR, HPO I/O will remain off if it was previously off.

[HOW]
Explicitly update HPO I/O after allocating encoders for test request.

Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Michael Strauss <michael.strauss@amd.com>
Signed-off-by: Roman Li <roman.li@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>