drm/msm/sde: move crtc towards multi-encoder support

SDE CRTC currently assumes one encoder per CRTC associated at
initialization time. Need to break this assumption and move
towards tracking multiple encoders, though full dynamic
assignment is not provided by this patch.

Change-Id: I47f42f5ef57f4df2f19f66bdef737cc86906ab1d
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>