drm/amd/display: early return when pipe_cnt is 0 in bw validation

[Why]
Unintentionally introduced behaviour change from previous refactor,
which causes clks to be 0 in no stream cases, which will cause
divide by 0.

[How]
Skip calculation of clocks when no stream. Which is the same as old
behaviour.
Signed-off-by: Eric Yang <Eric.Yang2@amd.com>
Reviewed-by: Tony Cheng <tong.cheng@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dcn20/dcn20_resource.c

@@ -2022,6 +2022,7 @@ bool dcn20_fast_validate_bw(
 		struct dc *dc,
 		struct dc_state *context,
 		display_e2e_pipe_params_st *pipes,
+		int *pipe_cnt_out,
 		int *pipe_split_from,
 		int *vlevel_out)
 {
@@ -2074,6 +2075,8 @@ bool dcn20_fast_validate_bw(
 		pipe_cnt = dcn20_populate_dml_pipes_from_context(dc,
 			&context->res_ctx, pipes);
 
+	*pipe_cnt_out = pipe_cnt;
+
 	if (!pipe_cnt) {
 		out = true;
 		goto validate_out;
@@ -2434,7 +2437,10 @@ bool dcn20_validate_bandwidth(struct dc *dc, struct dc_state *context,
 
 	BW_VAL_TRACE_COUNT();
 
-	out = dcn20_fast_validate_bw(dc, context, pipes, pipe_split_from, &vlevel);
+	out = dcn20_fast_validate_bw(dc, context, pipes, &pipe_cnt, pipe_split_from, &vlevel);
+
+	if (pipe_cnt == 0)
+		goto validate_out;
 
 	if (!out)
 		goto validate_fail;

drivers/gpu/drm/amd/display/dc/dcn20/dcn20_resource.h

@@ -120,6 +120,7 @@ bool dcn20_fast_validate_bw(
 		struct dc *dc,
 		struct dc_state *context,
 		display_e2e_pipe_params_st *pipes,
+		int *pipe_cnt_out,
 		int *pipe_split_from,
 		int *vlevel_out);
 void dcn20_calculate_dlg_params(