drm/amd/display: update link encoder assignment

drm/amd/display: update link encoder assignment

[Why]
Map a link encoder instance matching stream encoder instance
if possible.

[How]
Get the stream encoder instance and assign the same link
encoder instance if available.

Reviewed-by: PeiChen Huang <peichen.huang@amd.com>
Signed-off-by: Meenakshikumar Somasundaram <meenakshikumar.somasundaram@amd.com>
Signed-off-by: Wayne Lin <wayne.lin@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>