drm/amd/display: add missing null check

[why]
There is a coding error for a missing null check for stream pointer when iterating through
pipe_ctx.
Reviewed-by: Martin Leung <Martin.Leung@amd.com>
Acked-by: Jasdeep Dhillon <jdhillon@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dcn32/dcn32_hwseq.c

@@ -1303,7 +1303,7 @@ static void apply_symclk_on_tx_off_wa(struct dc_link *link)
 	if (link->phy_state.symclk_ref_cnts.otg > 0) {
 		for (i = 0; i < MAX_PIPES; i++) {
 			pipe_ctx = &dc->current_state->res_ctx.pipe_ctx[i];
-			if (pipe_ctx->stream->link == link && pipe_ctx->top_pipe == NULL) {
+			if (pipe_ctx->stream && pipe_ctx->stream->link == link && pipe_ctx->top_pipe == NULL) {
 				pipe_ctx->clock_source->funcs->program_pix_clk(
 						pipe_ctx->clock_source,
 						&pipe_ctx->stream_res.pix_clk_params,