OMAP4:DSS2: Avoid VID/GFX underflow errors

FIFO threshold calculations do not work for some scenarios.
Adjusted FIFO low threshold empirically to avoid underflow errors.

Change-Id: Ib979050acaeca206fa4ad26ed53659869495e010
Signed-off-by: Erik Gilling <konkers@android.com>
Signed-off-by: Lajos Molnar <lajos@ti.com>