drm: ati_pcigart: Do not access I/O MEM space using pointer derefs.

The PCI GART table initialization code treats the GART table mapping
unconditionally as a kernel virtual address.

But it could be in the framebuffer, for example, and thus we're
dealing with a PCI MEM space ioremap() cookie.  Treating that as a
virtual address is illegal and will crash some system types (such as
sparc64 where the ioremap() return value is actually a physical I/O
address).

So access the area correctly, using gart_info->gart_table_location as
our guide.

Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Dave Airlie <airlied@linux.ie>