drm/i915: Fix inconsistance between pfit.enable and scaler freeing

Despite that during hw readout we seem to have scalers assigned
to pipes, then call atomic_setup_scalers, at the commit stage in
skl_update_scaler there is a check, that if we have fb src and
dest of same size, we stage freeing of that scaler.

However we don't update pfit.enabled flag then, which makes
the state inconsistent, which in turn triggers a WARN_ON
in skl_pfit_enable, because we have pfit enabled,
but no assigned scaler.

To me this looks weird that we kind of do the decision
to use or not use the scaler at skl_update_scaler stage
but not in intel_atomic_setup_scalers, moreover
not updating the whole state consistently.

This fix is to not free the scaler if we have pfit.enabled
flag set, so that the state is now consistent
and the warnings are gone.

v2: - Put pfit.enable check into crtc specific place
      (Ville Syrjälä)

Bugzilla: https://gitlab.freedesktop.org/drm/intel/issues/577


Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Tomi Sarvela <tomi.p.sarvela@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200124172301.16484-1-stanislav.lisovskiy@intel.com