drm/msm/sde: correct crtc to use new plane state for update

Crtc atomic check is updating zpos of current plane state. This
would corrupt current plane state if the atomic check is initiated
by test only atomic commit.  In that case, the test only update
would propgate to current state incorrectly.

Modify atomic check to get new plane state and use this for
zpos update.

Change-Id: Ia7d8653e443c8ad3e3c684959083aeea25f1ff57
Signed-off-by: Alan Kwong <akwong@codeaurora.org>