disp: msm: sde: add helper function for setting crtc to conn state

Add changes to move the state operations for setting crtc to
connector state to a helper api. This can be reused during
preclose operation for issuing a drm commit.

Change-Id: I8daf4f2119c8d0e54c123a07255cb8119c2040ea
Signed-off-by: Jayaprakash <jmadiset@codeaurora.org>