drm/i915: Move scaler setup to check crtc function, v2.

The scaler setup may add planes, but since they're unchanged we only
have to wait for primary flips. Also set planes_changed to indicate
at least 1 plane is modified.

Changes since v1:
- Instead of removing planes, do minimal validation needed.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Tested-by(IVB): Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>