drm/amd/display: Drop legacy code

Display code keeps getting improvements, and because of that, some
legacy code is left behind. This commit drops some of those unused
codes.

Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>