drm/amd/display: add missing dcn link encoder regs

[Why]
The earlier change: "check phy dpalt lane count config"
uses link encoder registers not defined properly.
That caused regression with mst-enabled display not
lighting up.

[How]
Add missing reg definitions.

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