drm/v3d: fix a NULL vs error pointer mixup

The drm_gem_shmem_create() returns error pointers and v3d_bo_create() is
also supposed to return error pointers.

Fixes: 40609d48

 ("drm/v3d: Use the new shmem helpers to reduce driver boilerplate.")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20190321062731.GC21489@kadam