disp: msm: sde: fix reset of mixer swapping

This change resets the mixer swapping flag in crtc on a
seamless mode switch. In case of switching from command
mode to video mode it is a seamless mode on crtc, but we
disable connector and encoder. Due to this crtc fails to
reset the mixer swap since it is tied to dsc merge topology
on master connector. In transient state master connector
is not set and is set only after enabling the new mode on
connector.

Change-Id: Idebdabd642625b894100f62aeb62f11a9c101b03
Signed-off-by: Abhijit Kulkarni <kabhijit@codeaurora.org>