drm/i915: Use drm_atomic_get_existing_plane_state

We want to encapsulate the drm_atomic_state internals.

v2: Use intel_atomic_get_existing_plane_state (Maarten).

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1464818821-5736-3-git-send-email-daniel.vetter@ffwll.ch