drm/radeon/kms: fix gart setup on fusion parts (v2) backport

drm/radeon/kms: fix gart setup on fusion parts (v2) backport

BugLink: http://bugs.launchpad.net/bugs/788691

Backport of 8aeb96f80232e9a701b5c4715504f4c9173978bd
(drm/radeon/kms: fix gart setup on fusion parts (v2))
to the stable tree.

Out of the entire GART/VM subsystem, the hw designers changed
the location of 3 regs.

v2: airlied: add parameter for userspace to work from.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Steve Conklin <sconklin@canonical.com>