OMAPFB: use devm_kzalloc to allocate omapfb2_device

Use devm_kzalloc to allocate omapfb2_device. This fixes possible memory
leak:

drivers/video/omap2/omapfb/omapfb-main.c:2553 omapfb_probe() warn:
possible memory leak of 'fbdev'

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