staging: media: davinci_vpfe: Replace explicit NULL comparison with ! operator

This patch replaces the explicit NULL comparisons with ! operator.
It was found using Coccinelle:

@disable is_null@
expression e;
@@
- e == NULL
+ !e

Signed-off-by: sayli karnik <karniksayli1995@gmail.com>
Acked-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>