drm/omap: remove null test before kfree

Fix checkpatch warning:
WARNING: kfree(NULL) is safe this check is probably not required

Cc: David Airlie <airlied@linux.ie>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>