drm/msm: use old state & new state iterators

DRM framework was updated to support tracking of previous commit
state and next commit state for planes, crtcs and connectors in
order to prevent confusion.  This change updates msm_atomic.c to
use this new API.

Change-Id: I2f20d8ae7d51bae69fe719cb227bc61f796a0c3f
Signed-off-by: Steve Cohen <cohens@codeaurora.org>