drm/i915/ringbuffer: Set ring->gem_buffer = NULL on init unwind

The cleanup path for early abort failed to nullify the gem_buffer. The
likely consequence of this is zero, since a failure here should mean
aborting the module load.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>