agp/intel-agp: Clear entire GTT on startup

agp/intel-agp: Clear entire GTT on startup

commit fc61901373987ad61851ed001fe971f3ee8d96a3 upstream.

Some BIOSes fail to initialise the GTT, which will cause DMA faults when
the IOMMU is enabled. We need to clear the whole thing to point at the
scratch page, not just the part that Linux is going to use.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
[anholt: Note that this may also help with stability in the presence of
driver bugs, by not drawing to memory we don't own]
Signed-off-by: Eric Anholt <eric@anholt.net>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>