drm/nouveau: Validate channel indices passed from userspace.

When hacking the libdrm for improvements, I triggered a kernel crash
related to the fact that the NOUVEAU_NOTIFIEROBJ_ALLOC ioctl calls
nouveau_channel_get with an unchecked channel index.
The patch ensures that the channel index is an unsigned and validates
its value in nouveau_channel_get.

Signed-off-by: Michel Hermier <hermier@frugalware.org>
Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>