drm/atomic: Add connector atomic_check function, v2.

The atomic_check function is useful for implementing properties, but
it can be used for other connector modeset related checks as well.

Similar to plane check functions, on a modeset atomic_check() is always
called.

Changes since v1:
- Make sure atomic_check() is called on any modeset.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1491477543-31257-5-git-send-email-maarten.lankhorst@linux.intel.com