OMAPDSS: Improve fifo management code

OMAP4+ allows assigning the overlay FIFOs freely, but that is not
supported by omapdss yet. This patch takes a step forward by improving
the fifo management to be more flexible.

dispc.c is changed to keep track of the sizes of each fifo, and also the
overlay using each fifo.

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