drm/msm/dpu: remove encoder from crtc mixer struct

Not actively used. Clean up the crtc mixer struct.

changes in v2:
	- none

Signed-off-by: Jeykumar Sankaran <jsanka@codeaurora.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>