drm/i915: Ensure setting up scalers into staged crtc_state

From intel_atomic_check, call intel_atomic_setup_scalers() to
assign scalers based on staged scaling requests. Fail the
transaction if setup returns error.

Setting up of scalers should be moved to atomic crtc check  once
atomic crtc is ready.

v2:
-updated parameter passing to setup_scalers (me)

Signed-off-by: Chandra Konduru <chandra.konduru@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>