drm/radeon/kms: fix possible sigbus in evergreen accel code

drm/radeon/kms: fix possible sigbus in evergreen accel code

Not upstream in Linus's tree as it was fixed by a much more intrusive
patch in a different manner, commit c919b371cb734f42b11

2.6.35 and 2.6.36 do not contain blit support for evergreen
asics so if they use unmappable vram, you can end up with an
unreachable buffer address.  This should not be applied to drm-next
as that tree already contains evergreen blit support.  This should
only be applied to the 2.6.35 and 2.6.36 stable trees.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=30188

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Acked-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>