drm/msm/sde: support split display using single control path

This change adds support in encoders to use single control path
for split display use cases. Physical interface use the virtual
encoder's helper api to update the control path. Each physical
encoder only updates the active configuration in the control
path.

Change-Id: I9dc7013f8f11488b0ee6537385a58eb604f55fdc
Signed-off-by: Abhijit Kulkarni <kabhijit@codeaurora.org>