drm/amd/display: Added support for multiple eDP BL control

[WHY & HOW]
Driver currently assumes only 1 eDP is connected. Added support for
multiple eDP BL control.

Signed-off-by: Jake Wang <haonan.wang2@amd.com>
Acked-by: Bindu Ramamurthy <bindur12@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>