OMAPDSS: setup default fifo thresholds

At the moment we don't setup FIFO thresholds by default in omapdss. It's
supposed to be done by the user of omapdss. And that is missing from
omapdrm, causing unoptimal thresholds to be used when using omapdrm.

While I believe it's in theory better to allow the user of omapdss to
setup the fifo thresholds, in practice we always use the same values,
and we could as well setup the thresholds in omapdss.

Furthermore, in omapdss init we always swap the FIFO used for GFX and WB
overlays, but we don't swap the FIFO thresholds for those overlays
(which is the reason for omapdrm using unoptimal HW reset values). So
it would make sense to setup the thresholds to account for the swapping
of the FIFOs.

So, this patch adds code to setup default FIFO tresholds at omapdss
init.

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