drm/amd/display: Added multi instance support for ABM

[WHY & HOW]
ABM assumes only 1 eDP is connected. Refactored existing
ABM interface to support multiple instances.

Signed-off-by: Jake Wang <haonan.wang2@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Wayne Lin <waynelin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>