drm/atomic: pass old crtc state to atomic_begin/flush.

In intel it's useful to keep track of some state changes with old
crtc state vs new state, for example to disable initial planes or
when a modeset's prevented during fastboot.

Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
[danvet: squash in fixup for exynos provided by Maarten.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>