drm/radeon: silence out possible lock dependency warning

Silence out the lock dependency warning by moving bo allocation out
of ib mutex protected section. Might lead to useless temporary
allocation but it's not harmful as such things only happen at
initialization.

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