drm/amd/display: add a NULL pointer check

[Why & How]

We have to check if stream is properly initialized before calling
find_matching_pll(), otherwise we might end up trying to deferecence a
NULL pointer.

Cc: stable@vger.kernel.org # 6.1+
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Sung-huai Wang <danny.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dce112/dce112_resource.c

@@ -974,10 +974,12 @@ enum dc_status resource_map_phy_clock_resources(
 		|| dc_is_virtual_signal(pipe_ctx->stream->signal))
 		pipe_ctx->clock_source =
 				dc->res_pool->dp_clock_source;
-	else
-		pipe_ctx->clock_source = find_matching_pll(
-			&context->res_ctx, dc->res_pool,
-			stream);
+	else {
+		if (stream && stream->link && stream->link->link_enc)
+			pipe_ctx->clock_source = find_matching_pll(
+				&context->res_ctx, dc->res_pool,
+				stream);
+	}
 
 	if (pipe_ctx->clock_source == NULL)
 		return DC_NO_CLOCK_SOURCE_RESOURCE;