drm/amd/display: Fix USB4 null pointer dereference in update_psp_stream_config

[Why]
A porting error on a previous patch left the block of code that
causes the crash from a NULL pointer dereference.

More specifically, we try to access link_enc before it's assigned in
the USB4 case in the following assignment:

config.dio_output_idx = link_enc->transmitter - TRANSMITTER_UNIPHY_A;

[How]
That assignment occurs later depending on the ASIC version. It's only
needed on DCN31 and only after link_enc is already assigned.

Fixes: 98643044

 ("drm/amd/display: fix a crash on USB4 over C20 PHY")
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>