OMAPDSS: DISPC: Remove omap_dss_device pointer usage from dispc_mgr_pclk_rate()

The pixel clock rate for the TV manager is calculated by checking the device
type connected to the manager, and then requesting the VENC/HDMI interface for
the pixel clock rate.

Remove the use of omap_dss_device pointer from here by checking which interface
generates the pixel clock by reading the DSS_CTRL.VENC_HDMI_SWITCH bit.

Signed-off-by: Archit Taneja <archit@ti.com>