drm/atomic: Add macros to access existing old/new state, v2.

drm/atomic: Add macros to access existing old/new state, v2.

After atomic commit, these macros should be used in place of
get_existing_state. Also after commit get_xx_state should no longer
be used because it may not have the required locks.

The calls to drm_atomic_get_existing_$obj_state should no longer be
used, and converted over to these new calls.

Changes since v1:
- Expand commit message.
- Deprecate get_existing_*_state functions in the documentation.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1487256430-7625-4-git-send-email-maarten.lankhorst@linux.intel.com
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>