drm/i915: Remove use of crtc->config from i915_debugfs.c

crtc->config is updated to always contain to the active crtc_state
and only differs from crtc_state during crtc_disable. It will
eventually be removed, so start with some low hanging fruit.

For crtc->active the situation is the same; it will be removed
eventually. Instead use crtc->state->active.

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>