drm/atomic-helper: Do not call ->mode_fixup for CRTC which will be disabled

When a CRTC is going to be disabled, it's state may contain a display mode
with zeroed content.  This could be reproduced by HDMI cable hotplug out
operation with legacy fbdev support in dual display cases.  It would confuse
driver's CRTC callback ->mode_fixup and make the total state be rejected.
So, let's don't call the callback for the CRTC.

Signed-off-by: Liu Ying <gnuiyl@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1464341754-7087-1-git-send-email-gnuiyl@gmail.com