OMAP: DSS2: Clean up DISPC overlay manager register definitions

Represent manager/channel specific DISPC registers as inline functions returning
the required dispc_reg struct. This is done since the current method is not
scalable as the number of overlay managers increase in number.

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