OMAP: DSS2: OMAPFB: Reduce stack usage

omapfb_mode_to_timings() had struct fb_info, struct fb_var and struct
fb_ops allocated from stack. This caused the stack usage grow quite
high.

Use kzalloc to allocate the structs instead.

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