drm/amd/display: Add DCN3.1 DIO

Add support for the DIO (Display IO) block of DCN3.1 which controls
legacy HDMI/DP stream/link encoding.

HW Blocks:

    +--------+       +--------+
    |  DIO   |       |  DCCG  |
    +--------+       +--------+

Includes some updates to core logic for link encoder assignment and
future support for new high bandwidth output.

v2: squash in unused variable fix (Alex)

Acked-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>