drm: radeon: Use surface for PCI GART table.

This allocates a physical surface for the PCI GART table, this way no
matter what other surface configurations exist the GART table will
always be seen by the hardware properly.

We encode the file pointer of the virtual surface allocate using a
special cookie value, called PCIGART_FILE_PRIV.  On the last close, we
release that surface.

Just to be doubly safe, we run the pcigart table setup with the main
surface control register clear.

Based upon ideas from David Airlie and Ben Benjamin Herrenschmidt.

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