drm/vboxvideo: fix a NULL vs IS_ERR() check

The devm_gen_pool_create() function never returns NULL, it returns
error pointers.

Fixes: 4cc9b565

 ("drm/vboxvideo: Use devm_gen_pool_create")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211118111233.GA1147@kili