drm/amd/display: Use preferred DP link rate if specified

drm/amd/display: Use preferred DP link rate if specified

The DisplayPort code already has the concept of preferred link
settings, but it only allows setting a preferred lane count and
link width at the same time. It does not consider the possiblity
that some devices may not work on lower link rates but may
support various lane counts.

Allow specifying a preferred link rate which will be used as
the initial link rate when deciding the DP link settings.

This is necessary to support NUTMEG which only works with HBR
but not with RBR.

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

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>