OMAP: DSS2: OMAPFB: Fix invalid bpp for PAL and NTSC modes

omapfb_mode_to_timings() sets the bpp to 0 when bootarg omapfb.mode is set to
either "pal" or "ntsc". This patch corrects this by setting the bpp to 24, as
would be done if omapdss_default_get_recommended_bpp() would be called.

Signed-off-by: Maurus Cuelenaere <mcuelenaere@gmail.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>