[media] s5p-g2d: Fix NULL pointer warnings in g2d.c file

Fixes the following warnings detected by sparse:
warning: Using plain integer as NULL pointer

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>