drm/radeon/kms: Only restrict BO to visible VRAM size when pinning to VRAM.

This prevented radeon.test=1 from testing transfers from/to GTT beyond the
visible VRAM size.

Signed-off-by: Michel Dänzer <daenzer@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>