[media] davinci: vpif: fix return value check for vb2_dma_contig_init_ctx()

In case of error, the function vb2_dma_contig_init_ctx() returns
ERR_PTR() and never returns NULL. The NULL test in the return value
check should be replaced with IS_ERR().
dpatch engine is used to auto generate this patch.
(https://github.com/weiyj/dpatch

)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Lad, Prabhakar <prabhakar.lad@ti.com>
[prabhakar.lad@ti.com: reword commit header]
Signed-off-by: Lad, Prabhakar <prabhakar.lad@ti.com>

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>