OMAPDSS: Use WB fifo for GFX overlay

OMAPDSS: Use WB fifo for GFX overlay

OMAP4's GFX overlay has smaller fifo than the rest of the overlays
(including writeback "overlay"). This seems to be the reason for
underflows in some more demanding scenarios.

We can avoid the problems by using the WB fifo for GFX overlay, and vice
versa. WB usage is not supported yet, but when it will, it should
perform just fine with smaller fifo as there are no hard realtime
constraints with WB.

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