agp/intel-gtt: Remove get/put_pages

If a page isn't allocated as __GFP_MOVEABLE it won't move around, so
no need to grab a reference to lock it into place.

Discovered while reviewing page allocation handling in i915 gem.

Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>