OMAPDSS: DISPC: fix DS variable name

check_horiz_timing_omap3() has a variable named 'DS'. i386 uses DS name
for something else, causing a compilation error. As 'DS' is not a very
good local variable name in the first place, let's change it to 'ds',
fixing the issue.

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