drm: virtio: fix kmem_cache_alloc error check

kmem_cache_alloc returns NULL on error, not ERR_PTR.

Fixes: f5985bf9
Reported-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1489393346-13874-1-git-send-email-kraxel@redhat.com