drm/i915: Move plane_state->scaler_id initialization into intel_create_plane_state()

No point in having each caller of intel_create_plane_state() initialize
the scaler_id to -1. Instead just do it in intel_create_plane_state().

Previously we left scaler_id at 0 for pre-SKL platforms, but I can't
see how initializing it to -1 always would cause any harm.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181005125817.22576-8-ville.syrjala@linux.intel.com