drm/radeon/kms: rewrite DP handling

- reorganize the functions based on use
- clean up function naming
- rework link training to better match what we use internally
- add initial support for DP 1.2 (no MST yet)

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>