drm/i915/bdw: Make sure error capture keeps working with Execlists

Since the ringbuffer does not belong per engine anymore, we have to
make sure that we are always recording the correct ringbuffer.

TODO: This is only a small fix to keep basic error capture working, but
we need to add more information for it to be useful (e.g. dump the
context being executed).

v2: Reorder how the ringbuffer is chosen to clarify the change and
rename the variable, both changes suggested by Chris Wilson. Also,
add the TODO comment to the code, as suggested by Daniel.

Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>