OMAPDSS: APPLY: write fifo thresholds only if changed

OMAPDSS: APPLY: write fifo thresholds only if changed

Current code will always write fifo threshold values to the register,
even if they are the same as previously. Separate the setting of fifo
fields into a separate function, and only set new values if they are
different than the old ones.

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