drm/radeon: allocate semaphore from the ib pool

This allow to share the ib pool with semaphore and avoid
having more bo around.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>