drm/i915: Handle error to get connector state when staging config

The return value of one of the calls to drm_atomic_get_connector_state()
in intel_modeset_stage_output_state() wasn't checked for errors.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>