drm/amd/display: Do not skip link training on DP quick hot plug

[Why]
When rapidly plugging and unplugging a DP sink, detection link
training can be mistakenly skipped.

This is due to the hotplug processing occurring before the
encoder assignment logic has had a chance to process the removal
of a stream. The encoder that would be used for detection link
training is then erroneously reported as already in use and
detection link training is skipped.

[How]
During hot plug processing, only determine a link encoder to be
unavailable for a particular link if it has been assigned to a
different link.

Reviewed-by: Meenakshikumar Somasundaram <Meenakshikumar.Somasundaram@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Agustin Gutierrez Sanchez <agustin.gutierrez@amd.com>
Signed-off-by: Jimmy Kizito <Jimmy.Kizito@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>