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: Ic29f86a75d8fe97178f86d8cbe12e7984f942deb
Signed-off-by: Mahadevan <mahap@codeaurora.org>