drm/amd/display: Use HBR2 if eDP monitor it doesn't advertise link rate

Some eDP displays use the extra link rate table to advertise link rate
support. If they do that they don't need to provide link rate through
the usual registers. Since we don't currently have support for the extra
link rate table default to HBR2 for the display in this.

Note that this is a HACK. Ultimately we need to teach DC to use the
extra link rate table.

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>