media: venus: venc: Fix potential null pointer dereference on pointer fmt

Currently the call to find_format can potentially return a NULL to
fmt and the nullpointer is later dereferenced on the assignment of
pixmp->num_planes = fmt->num_planes.  Fix this by adding a NULL pointer
check and returning NULL for the failure case.

Addresses-Coverity: ("Dereference null return")

Fixes: aaaa93ed ("[media] media: venus: venc: add video encoder files")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>