drm/amd/display: Fix overlay with pre-blend color processing

[WHY]
Overlay works similarly to MPO, but uses global alpha on both planes and
sets the desktop as the rear plane instead of the front plane

[HOW]
Ensure that top plane isn't overlay by checking global alpha before
applying the previously added MPO fix

Reviewed-by: Rodrigo Siqueira <[e-mail]>
Signed-off-by: Michael Strauss <[e-mail]>
Signed-off-by: Wayne Lin <[e-mail]>
Tested-by: Daniel Wheeler <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/display/dc/hwss/dcn10/dcn10_hwseq.c

···
 
 			while (top->top_pipe)
 				top = top->top_pipe; // Traverse to top pipe_ctx
-			if (top->plane_state && top->plane_state->layer_index == 0)
-				return true; // Front MPO plane not hidden
+			if (top->plane_state && top->plane_state->layer_index == 0 && !top->plane_state->global_alpha)
+				// Global alpha used by top plane for PIP overlay
+				// Pre-multiplied/per-pixel alpha used by MPO
+				// Check top plane's global alpha to ensure layer_index > 0 not caused by PIP
+				return true; // MPO in use and front plane not hidden
 		}
 	}
 	return false;