drm/atomic: Make mode_fixup() optional for check_modeset()

So the i915 driver can use the same logic for setting mode and active
changed flags, without having to implement encoder helpers and the
mode_fixup() callback.

Cc: dri-devel@lists.freedestkop.org
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>