OMAP2: DSS: Add missing line for update bg color

The driver set the background color canvas but never writes it in
DISPC_DEFAULT_COLOR_m register, which changes the background color
on the LCD or TV.

This patch adds a line to call to dispc_set_default_color() which is
the function in charge to write the DISPC_DEFAULT_COLOR_m register.

Signed-off-by: Carlos Lopez <carlos.cruz@sasken.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>