OMAPDSS: DISPC: Add naive threshold calc for fifomerge

Take fifo merge into use by implementing a rather naive fifo merge
threshold calculation: keep the low threshold always the same, but
increase the high threshold when fifo merge is used.

This should greatly increase the time between pixel data fetches from
SDRAM, as the usable fifo size is much larger. However, it probably
won't help for fifo underflows, as the low threshols is kept the same.

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