drm/amd/display: Hook DCN2 into amdgpu_dm and expose as config (v2)

Enable DCN2 support in DM (Display Manager).

v2: fix spurious raven change (Alex)

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>