drm/amd/display: Don't skip link training for empty dongle

[Why]
Skipping initial link training will result in no verified link cap for
mode enumeration. Some versions of the BIOS seem to have PHY programming
sequence issue as well if initial link training is skipped, resulting in
a softlock in BIOS command table.

[How]
Identify the empty dongle hotplug case, and still do initial link
training.

Signed-off-by: Eric Yang <Eric.Yang2@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>