drm/i915: Include a little more information about why ring init fails

If we include the expected values for the failing ring register checks,
it makes it marginally easier to see which is the culprit.

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