agp: add agp_generic_destroy_pages()

Add agp_generic_destroy_pages(), it uses new pageattr array interface API.

Signed-off-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>