drm/i915: Do not update pfit state when toggling crtc enabled.

There's not much point for calculating the changes for the old
state. Instead just disable all scalers when disabling. It's
probably good enough to just disable the crtc_scaler, but just in
case there's a bug disable all scalers.

This means intel_atomic_setup_scalers is only called in the crtc
check function now, so all the transitional code can be removed.

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>