drm/radeon/dp: add back special handling for NUTMEG

drm/radeon/dp: add back special handling for NUTMEG

When I fixed the dp rate selection in:
092c96a8ab9d1bd60ada2ed385cc364ce084180e
drm/radeon: fix dp link rate selection (v2)
I accidently dropped the special handling for NUTMEG
DP bridge chips.  They require a fixed link rate.

Reviewed-by: Christian König <[email protected]>
Reviewed-by: Ken Wang <[email protected]>
Reviewed-by: Harry Wentland <[email protected]>
Tested-by: Ken Moffat <[email protected]>
Signed-off-by: Alex Deucher <[email protected]>
Cc: [email protected]