drm/msm/sde: use new iterator for connectors

DRM framework has deprecated drm_for_each_connector and now provides
an interator instead.  This change adds the corresponding changes
in the SDE sub-driver.

Change-Id: I8db60513223df70b1c560183636adb5411877bd3
Signed-off-by: Steve Cohen <cohens@codeaurora.org>