i915: support 8xx desktop cursors

For some reason we never added 8xx desktop cursor support to the
kernel.  This patch fixes that.

[krh: Also set the size on pre-i915 hw.]
Tested-by: Kristian Høgsberg <krh@redhat.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>