Use the fence of the new frontbuffer, if any.

Generating a new fence could cause us to wait for completely unrelated
rendering to finish before performing the flip.
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>