drm/amd/display: Implement BIOS parser external encoder control

The VBIOS has a function called ExternalEncoderControl which
controls the DP bridge encoders that some GPUs use for analog
and LVDS output. Fixup this old functionality.

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

- Set same parameters for the ENABLE action as the SETUP action
- Add missing enum values for DDC setup and DAC load detection
- Fix the bits per color field
- Clarify the code that sets the link rate
- Expose the function so that it can be called by rest of DC

A subsequent commit will call this function from DCE HWSS.

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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