drm/radeon: prevent crash in the ring space allocation

drm/radeon: prevent crash in the ring space allocation

commit fd5d93a0015ce1a7db881382022b2fcdfdc61760 upstream.

If the requested number of DWs on the ring is larger than
the size of the ring itself, return an error.

In testing with large VM updates, we've seen crashes when we
try and allocate more space on the ring than the total size
of the ring without checking.

This prevents the crash but for large VM updates or bo moves
of very large buffers, we will need to break the transaction
down into multiple batches.  I have patches to use IBs for
the next kernel.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
[bwh: Backported to 3.2: use rdev->cp.ring_size instead of ring->ring_size]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>