Since writeback has many overlay like properties, and most of it's registers are
similar to that of overlays, it's possible to reuse most of the overlay related
DISPC code for writeback when considering it as a plane. Writeback was added as
a plane in the omap_plane field as OMAP_DSS_WB.

Add the writeback register offsets in dispc.h, add minimal WB plane related info
needed in dss_features. Add a function which returns the number of writeback
pipelines an OMAP version has.
Signed-off-by: Archit Taneja <archit@ti.com>