drm/amd/display: Link detection for external DP bridge encoders

drm/amd/display: Link detection for external DP bridge encoders

Deal with some minor idiosyncracies of TRAVIS and NUTMEG chips.

- Always use DP signal type with these chips so that the
  normal DP code paths can work with them without a major
  refactor of the code base. Properly set this.

- NUTMEG seems to only work with HBR, not RBR, so set a
  preferred link rate for this chip.
  See amdgpu_atombios_dp_get_dp_link_config() for reference.

- NUTMEG is recognized as a DP branch device but reports 0 sinks,
  which is wrong and confuses DC (it hits an early return).
  Fix that by hardcoding the sink count to 1.

- Fixup old DC code selecting a special panel mode necessary
  for NUTMEG and TRAVIS.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>