drm/amd/display: Fix surface optimization regression on Carrizo

[Why]
DCE legacy optimization path isn't well tested under new DC optimization
flow which can result in underflow occuring when initializing X11 on
Carrizo.

[How]
Retain the legacy optimization flow for DCE and keep the new one for DCN
to satisfy optimizations being correctly applied for ASIC that can
support it.

Fixes: 34316c1e ("drm/amd/display: Optimize bandwidth on following fast update")
Reported-by: Tom St Denis <tom.stdenis@amd.com>
Tested-by: Tom St Denis <tom.stdenis@amd.com>
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.c

@@ -3118,8 +3118,13 @@ void dc_commit_updates_for_stream(struct dc *dc,
 			if (new_pipe->plane_state && new_pipe->plane_state != old_pipe->plane_state)
 				new_pipe->plane_state->force_full_update = true;
 		}
-	} else if (update_type == UPDATE_TYPE_FAST) {
-		/* Previous frame finished and HW is ready for optimization. */
+	} else if (update_type == UPDATE_TYPE_FAST && dc_ctx->dce_version >= DCE_VERSION_MAX) {
+		/*
+		 * Previous frame finished and HW is ready for optimization.
+		 *
+		 * Only relevant for DCN behavior where we can guarantee the optimization
+		 * is safe to apply - retain the legacy behavior for DCE.
+		 */
 		dc_post_update_surfaces_to_stream(dc);
 	}
 
@@ -3178,6 +3183,12 @@ void dc_commit_updates_for_stream(struct dc *dc,
 		}
 	}
 
+	/* Legacy optimization path for DCE. */
+	if (update_type >= UPDATE_TYPE_FULL && dc_ctx->dce_version < DCE_VERSION_MAX) {
+		dc_post_update_surfaces_to_stream(dc);
+		TRACE_DCE_CLOCK_STATE(&context->bw_ctx.bw.dce);
+	}
+
 	return;
 
 }