OMAP: DSS2: DSI: increase FIFO low threshold

The current FIFO low threshold was too low, and caused the FIFO to run
empty when core domain went to INA state between FIFO fills. This patch
increases the low threshold to keep that from happening.

The threshold values depend quite much on the HW and the use cases, so
this should actually be somehow configurable from board files, perhaps.

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