drm/amd/display: Update display endpoint control path.

[Why]
Some display endpoints may be dynamically mapped to the link encoders
which drive them.

[How]
Update the code paths for display enabling/disabling to accommodate
the dynamic association between links and link encoders.

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