drm/amd/display: Move link_enc init logic to DC

[Why]

We shouldn't be accessing res_pool funcs from DM level,
therefore, we should create API and let the flow
be done in DC level.

[How]

We create new interface dp_get_link_enc to access and get the correct link_enc

Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Pavle Kotarac <Pavle.Kotarac@amd.com>
Signed-off-by: Jude Shih <shenshih@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>