drm/omap: Store pixel clock instead of full mode in DPI and SDI encoders

The DPI and SDI encoders store the full videomode upon mode set, to only
use the value of the pixel clock when enabling the encoder. This wastes
memory. Store the pixel clock value only.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Tested-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>