drm/amd/display: prepare for new platform

[Why & How]
Expose some function for new platform use

Reviewed-by: Nevenko Stupar <nevenko.stupar@amd.com>
Signed-off-by: Karthi Kandasamy <karthi.kandasamy@amd.com>
Signed-off-by: Ray Wu <ray.wu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>