1. A power management device for a computing system, the computing system comprising: a multi-GPU graphics processing and display subsystem having at least one internal GPU and at least one external GPU and supporting multi-GPU application-division parallel modes of graphics processing in order to execute graphic commands and process graphics data and render pixel-composited images containing graphics for display on a display device during the run-time of the multiple graphics-based applications; and an automatic mode control module for automatically controlling the mode of operation of the multi-GPU graphics processing and display subsystem during the run-time of the multiple graphics-based applications so as to ensure efficient power management and graphics processing resource allocation; wherein the automatic mode control module employs real-time detection of individual application profiles and determines which GPU or GPUs in the multi-GPU graphics processing and display subsystem should be assigned to process the stream of graphics command and data of particular graphics applications running on the computer system; wherein for an unknown graphics based application which has not been previously profiled for graphics processing requirements, the automatic mode control module automatically reads an initialization header of the graphics based application to determine whether the unknown graphics based application is a low graphics based application or a high graphics based application, and based on such determination automatically send the graphics commands and data associated with the unknown graphics based application to the at least one internal GPU or the at least one external GPU; and wherein a power management module in the subsystem, in response to the determination as to which GPU or GPUs are assigned to process, selectively puts none, one or more than one of the multiple GPUs into an idle state to manage and conserve power.