OMAP: DSS2: Clean up DISPC overlay register definitions

Move all DISPC register definitions to a new header dispc.h. There are
separate register offset definitions for GFX, VID1 and VID2 pipeline share
register definitions by using an argument. The introduction of VID3 pipeline
on OMAP4 will not let us use the above method since VID3 pipe register offsets
don't map with VID1 and VID2 offsets.

Represent overlay registers as DISPC_OVL_XXXX(plane), where the plane argument
tells the overlay. Register offsets are calculated as:

DISPC_OVL_XXXX(plane) = DISPC_OVL_BASE(plane) + DISPC_XXXX_OFFSET(plane)

Idea suggested by Tomi Valkeinen.

Signed-off-by: Archit Taneja <archit@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>