drm/i915: Remove crtc->config usage from intel_modeset_readout_hw_state()

crtc->config is on its way out. Let's reduce our dependence on it a
little bit by removing it from intel_modeset_readout_hw_state().

Also replace crtc->acttive checks with crtc_state->base.active checks.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Suggested-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161222140442.7204-1-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>