drm/i915: Use atomic state to obtain load detection crtc, v3.

Instead of restoring dpms and a flag for whether a temp fb is allocated duplicate
an atomic state before the new state is committed, and commit it the old state
in intel_release_load_detect_pipe.

Changes since v1:
- Use a real atomic state. (Ville)
Changes since v2:
- Do not preserve shared_dpll any more, no need to do so. (Ville)

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1455697119-31416-2-git-send-email-maarten.lankhorst@linux.intel.com