drm/amd/display: prepare for new platform

Expose some functions for new platform use

Reviewed-by: Nevenko Stupar <nevenko.stupar@amd.com>
Signed-off-by: Karthi Kandasamy <karthi.kandasamy@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>