[media] media: davinci: vpbe: fix return value check in vpbe_display_reqbufs()

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().

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Prabhakar Lad <prabhakar.lad@ti.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>