char/agp: consolidate {alloc,free}_gatt_pages()

There is a copy of alloc_gatt_pages() and free_gatt_pages in several
architectures in arch/$ARCH/include/asm/agp.h. All the copies do exactly
the same: alias alloc_gatt_pages() to __get_free_pages(GFP_KERNEL) and
alias free_gatt_pages() to free_pages().

Define alloc_gatt_pages() and free_gatt_pages() in drivers/char/agp/agp.h
and drop per-architecture definitions.

Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>