drm/i915: Make HW readout mark CRTC scaler as in use.

This way we don't accidentally double allocate it.
Noticed this when I wrote a patch to sanity check all of
the scaler state.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190108160842.13396-4-maarten.lankhorst@linux.intel.com