[PATCH] drm/nv50-nvc0: work around an evo channel hang that some people see

[Seth Forshee]

BugLink: http://bugs.launchpad.net/bugs/583760

SRU Justification

Impact: Mouse cursor disappears after some time of using the nouveau
driver on a computer with NV50 graphics

Fix: Backport of upstream workaround that avoids using the last 32 bytes
of the evo ring buffer.

Test case: Verified against natty on LP #583760.

Note that no one has stepped up to verify this patch on lucid or
maverick. However, since the patch is simple and unlikely to cause
regressions I'm submitting it for those releases as well.