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>

drivers/gpu/drm/amd/display/dc/core/dc_link.c

@@ -4009,12 +4009,9 @@ static void update_psp_stream_config(struct pipe_ctx *pipe_ctx, bool dpms_off)
 		config.dig_be = pipe_ctx->stream->link->link_enc_hw_inst;
 #if defined(CONFIG_DRM_AMD_DC_DCN)
 		config.stream_enc_idx = pipe_ctx->stream_res.stream_enc->id - ENGINE_ID_DIGA;
-		
+
 		if (pipe_ctx->stream->link->ep_type == DISPLAY_ENDPOINT_PHY ||
 				pipe_ctx->stream->link->ep_type == DISPLAY_ENDPOINT_USB4_DPIA) {
-			link_enc = pipe_ctx->stream->link->link_enc;
-			config.dio_output_type = pipe_ctx->stream->link->ep_type;
-			config.dio_output_idx = link_enc->transmitter - TRANSMITTER_UNIPHY_A;
 			if (pipe_ctx->stream->link->ep_type == DISPLAY_ENDPOINT_PHY)
 				link_enc = pipe_ctx->stream->link->link_enc;
 			else if (pipe_ctx->stream->link->ep_type == DISPLAY_ENDPOINT_USB4_DPIA)