drm/i915: Read hw state into an atomic state struct, v2.

To make this work we load the new hardware state into the
atomic_state, then swap it with the sw state.

This lets us change the force restore path in setup_hw_state()
to use a single call to intel_mode_set() to restore all the
previous state.

As a nice bonus this kills off encoder->new_encoder,
connector->new_enabled and crtc->new_enabled. They were used only
to restore the state after a modeset.

Changes since v1:
- Make sure all possible planes are added with their crtc set,
  so they will be turned off on first modeset.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>