OMAPDSS: Add struct to hold LCD overlay manager configuration

Create a struct dss_lcd_mgr_config which holds LCD overlay manager related
parameters. These are currently partially contained in the omap_dss_device
connected to the manager, and the rest are in the interface driver.

The parameters are directly written to the DISPC registers in the interface
drivers. These should eventually be applied at the correct time using the
shadow register programming model. This struct would help in grouping these
parameters so that they can be applied together.

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