x86/mm/gart: Drop unnecessary check

The memblock_find_in_range() return value addr is guaranteed
to be within "addr + aper_size" and not beyond GART_MAX_ADDR.

Signed-off-by: Wang YanQing <udknight@gmail.com>
Cc: yinghai@kernel.org
Link: http://lkml.kernel.org/r/20130416013734.GA14641@udknight
Signed-off-by: Ingo Molnar <mingo@kernel.org>