intel: Mark cached bo as purgeable

Set the DONTNEED flag on cached buffers so that the kernel is free to
discard those when under memory pressure.

[anholt: This takes firefox-talos-gfx time from ~62 seconds to ~65 seconds
on my GM965, but it seems like a hit worth taking for the improved
functionality from saving memory]

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>