drm/armada: move sync signal polarity to mode_set_nofb() method

For atomic modeset, we need to set the sync signal polarities from the
CRTC state structure rather than the legacy mode structure stored in
CRTC.  In any case, we should update this from our mode_set_nofb()
method, rather than the commit() method.  Move it there, and ensure
that armada_drm_crtc_update() will not overwrite these bits.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>