media: omap2: omapfb: fix ifnullfree.cocci warnings

drivers/video/fbdev/omap2/omapfb/dss/core.c:141:2-26: WARNING: NULL check before some freeing functions is not needed.

 NULL check before some freeing functions is not needed.

 Based on checkpatch warning
 "kfree(NULL) is safe this check is probably not required"
 and kfreeaddr.cocci by Julia Lawall.

Generated by: scripts/coccinelle/free/ifnullfree.cocci

Signed-off-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>