drm/amd/display: Fix MST link encoder availability check.

[Why]
MST streams share the same link and should share the same encoder.
The current availability check may erroneously determine that an
encoder is unavailable for MST streams.

[How]
When checking for link encoder availability, check if an encoder
in use shares a link with the stream for which the availability
check is being conducted. If the link is shared, then the link
encoder should be shared too and will be deemed available.

Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@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>