drm/i915: Use pipe_name() in the get_plane_config() functions

We may as well try to be consistent everywhere and know the pipes by
their name.

Reviewed-By: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>