drm/amd/display: USB-C / thunderbolt dock specific workaround

reading dpcd 0x600 cause link loss for a particular USB-C dock with
thurderbolt.  workaround by avoiding dcpd 0x600 read unless it's
necessary.

Signed-off-by: Hersen Wu <hersenxs.wu@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>