drm/msm/sde: cleanup encoder implementation

This code cleans up video and cmd mode encoders by moving
the implementation of identifying single ctrl into common
header file and waits for LUTDMA flush only once for single
control path configuration with two interfaces.

Change-Id: Ib1978540dc3ebc7e4866fb4844456b171a830c3b
Signed-off-by: Abhijit Kulkarni <kabhijit@codeaurora.org>