drm/amd/display: Test HPD RX offload work branches

[WHAT]
dm_handle_hpd_rx_offload_work() was covered only for its
automated-test and link-loss branches. The connection-type failure, the
GPU-reset drop, the MST message-ready drain and the forced reconnect
were never entered.

[HOW]
Add a fixture that builds a queued offload work item on a
DisplayPort link and four tests: a failing connection-type probe, a
device already in reset, a deferred UP_REQ_MSG_RDY drained through an
AUX that fails every transfer, and an automated test whose timing
changed, which forces the connector off and back on. The last one
registers the DRM device so the hotplug uevent helpers find a sysfs
minor on the primary node.

Assisted-by: Copilot:Claude-Opus-5
Reviewed-by: Bhawanpreet Lakha <bhawanpreet.lakha@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: George Zhang <george.zhang@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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