drm/amd/display: Update link encoder object creation

[Why]
Currently the creation of link encoder objects is tightly coupled to the
creation of link objects. Decoupling link encoder object creation is a
preliminary step in the process of allowing link encoders to be
dynamically assigned to links.

[How]
Add "minimal" link encoder objects which are not associated with any
link until required.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Jimmy Kizito <Jimmy.Kizito@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>