[media] s5p-mfc: Add NULL check for allocated buffer

In certain cases, dma_alloc_coherent returns NULL. Add check for
NULL pointer.

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