drm/amd/display: skip retrain in dc_link_set_preferred_link_settings() if using passive dongle

[Why]
Fixes issue when we have a display connected using a passive
dongle and then emulate over it using a DP connection at 1 x 1.62 Ghz.
System hangs because register bus returns back 0xFFFFFFFF for all
register reads after setting register DIG_BE_CNTL in
dcn10_link_encoder_connect_dig_be_to_fe().  Hang occurs later
when trying to do a register read.

[How]
At the start of the emulation, dc_link_set_preferred_link_settings()
and dp_retrain_link_dp_test() is called, even though it is connected
using a passive dongle.

Add an extra condition in dp_retrain_link_dp_test() to check for
link->dongle_max_pix_clk > 0.  This is the only way we know if the
connection is using passive dongle so we don't retrain DP.

Signed-off-by: Samson Tam <Samson.Tam@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>