[media] DaVinci-VPIF-Display: Adjust 11 checks for null pointers

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: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>