Create struct omap_dss_writeback_info, this is similar to omap_overlay_info,
the major difference is that there is no parameter which describes the input
size to writeback, this is because this is always fixed, and decided by the
connected overlay or overlay manager. One more difference is that screen_width
is renamed to buf_width, to give the value of stride the writeback buffer has.

Call dispc_ovl_setup_common() through dispc_wb_setup() to configure overlay-like
parameters. The parameters in dispc_ovl_setup_common() which do not hold for
writeback are filled passed as zeroes or false, the code takes care of not
configuring them as they won't possess the needed overlay caps.

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