drm/msm/sde: connect physical encoder to debugfs

Physical encoder is currently not connected to
debugfs. Connect physical encoders to debugfs by
calling late_register in virtual encoder.

CRs-Fixed: 2030127
Change-Id: Ic681171a9a43962cfa875235c769994fef8b9aed
Signed-off-by: Alan Kwong <akwong@codeaurora.org>