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

When I fixed the dp rate selection in:
092c96a8
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 <christian.koenig@amd.com>
Reviewed-by: Ken Wang <Qingqing.Wang@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Tested-by: Ken Moffat <zarniwhoop@ntlworld.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org