drm/i915: Allow the module to load even if we fail to setup rings

Even without enabling the ringbuffers to allow command execution, we can
still control the display engines to enable modesetting. So make the
ringbuffer initialization failure soft, and mark the GPU as wedged
instead.

v2: Only treat an EIO from ring initialisation as a soft failure, and
abort module load for any other failure, such as allocation failures.

v3: Add an *ERROR* prior to declaring the GPU wedged so that it stands
out like a sore thumb in the logs

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Oscar Mateo <oscar.mateo@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>