OMAPDSS: add 'has_writeback' flag

At the moment we have a function to get the number of writeback
pipelines supported. However, the function is used in a wrong way,
causing a wrong pipeline to get configured when trying to use WB. Also,
we only have a single writeback pipeline on any of the DSS versions.

To fix and simplify this, create a 'has_writeback' flag into the dispc
driver, and after checking the flag, use OMAP_DSS_WB pipeline
explicitly when calling the configuration functions.

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