drm/radeon: properly program gart on rv740, juniper, cypress, barts, hemlock

commit 0b8c30bc4943137a4a36b9cb059b1cc684f5d702 upstream.

Need to program an additional VM register.  This doesn't not currently
cause any problems, but allows us to program the proper backend
map in a subsequent patch which should improve performance on these
asics.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>