drm/amd/display: Implement DAC load detection on external DP bridge encoders

drm/amd/display: Implement DAC load detection on external DP bridge encoders

Use the pre-existing implementation in the BIOS parser, but call
the ExternalEncoderControl function for external encoders instead
of the built-in DAC load detection function.

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>