drm/msm/sde: sde encoder virtualization

Split SDE encoder into virtual and physical encoders. Virtual
encoders are containers, one per logical display that contain
one or more physical encoders. Physical encoders manage the
INTF hardware.

Change-Id: I6342511c59568c76278a519b84f93338157e59fa
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>
Signed-off-by: Krishna Srinivas Kundurthi <kskund@codeaurora.org>