drm/i915: Fix reference leak in intel_modeset_readout_hw_state.

Unreference the old mode_blob by calling the crtc_destroy_state
helper before zeroing the crtc_state.

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>