drm/atomic: Clean up steal_encoder, v2.

Now that only encoders can be stolen that are part of the state
steal_encoder no longer needs to inspect all connectors,
just those that are part of the atomic state.

Changes since v1:
- Change return value to void, can no longer fail.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[danvet: Appease gcc, reported by kbuild.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1456996662-8704-7-git-send-email-maarten.lankhorst@linux.intel.com