OMAPFB: fix error handling in omapfb_find_best_mode()

omapfb_find_best_mode() doesn't check for the return value of kmalloc.
Fix this. This also removes the smatch warning:

drivers/video/omap2/omapfb/omapfb-main.c:2256 omapfb_find_best_mode()
error: potential null dereference 'specs'.  (kzalloc returns null)

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