drm/i915: Preserve scaler state when clearing crtc_state

crtc_state is cleared during mode set which wipes out complete
scaler state too. This is causing issues. To fix, ensure scaler
state is preserved because it contains not only crtc
scaler usage, but also planes using scalers on this crtc.

Signed-off-by: Chandra Konduru <chandra.konduru@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>