intel-gtt: generic (insert|remove)_entries for i830

Well, not all too generic because it does not yet support dmar.
Add a new function check_flags to ensure that non-gem code does
not try to screw us over.

v2: Beautify i830_check_flags with an idea from Chris Wilson.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>