drm/amd/display: Add DCE HWSS support for external DP bridge encoders

Some GPUs use external DP bridge encoders NUTMEG and TRAVIS
to implement analog and/or LVDS connections. Typically found in
CIK APU based laptops or on FM2 motherboards that have analog
connectors. These were necessary at the time because Kaveri
didn't have a built-in DAC nor LVDS support.

These devices sadly don't work transparently and need to be
controlled by the driver. Implement the necessary control for
the NUTMEG and TRAVIS encoders in the DCE HWSS.

For reference, see the legacy non-DC amdgpu display code:
amdgpu_atombios_encoder_setup_external_encoder()
amdgpu_atombios_encoder_setup_dig()
amdgpu_atombios_encoder_setup_ext_encoder_ddc()

- Prepare DDC before using it:
  Call the EXTERNAL_ENCODER_CONTROL_DDC_SETUP action so that
  the encoder knows to set up DDC over the AUX channel.

- When a stream is enabled or disabled:
  Call the EXTERNAL_ENCODER_CONTROL_ENABLE/DISABLE actions.

- Before enabling the DP link:
  Call the EXTERNAL_ENCODER_CONTROL_SETUP action.

This commit just hooks up the HWSS support.
Detecting the external DP bridge encoders will be done in
a subsequent commit.

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>

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