drm/radeon/kms: force pinning buffer into visible VRAM

This patch properly set visible VRAM and enforce any pinned buffer
to be into visible VRAM. We might later add a flag to release this
constraint for some newer hw more clever than previous.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>