drm/i915: Clean up intel_atomic_setup_scalers slightly.

Get rid of a whole lot of ternary operators and assign the index
in scaler_id, instead of the id. They're the same thing.

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>