drm/i915: Update power domains on readout.

This allows us to get rid of the set_init_power in
modeset_update_crtc_domains. The state should be sanitized enough
after setup_hw_state to not need the init power.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>