drm/i915: Use atomic state in intel_fb_initial_config.

This is another step in removing legacy state.

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/1455697119-31416-6-git-send-email-maarten.lankhorst@linux.intel.com