drm/amd/display: Add dp_skip_rbr flag for NUTMEG

drm/amd/display: Add dp_skip_rbr flag for NUTMEG

No functional changes. Just clean up a conceptual mismatch.

Based on feedback on the NUTMEG code in DC, the
preferred_link_setting is meant to force the DP link to a
specific setting, meaning both the link rate and lane count
should be locked to an exact value. What NUTMEG needs is
a lower bound on the link rate, which is not the same concept.

Implement this as a HW workaround flag instead.

Suggested-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>