media: s5p-mfc: Adjust a null pointer check in four functions

The script “checkpatch.pl” pointed information out like the following.

Comparison to NULL could be written...

Thus fix the affected source code places.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>