drm/i915: Fix up the inverse mapping for default ctx->engines[]

The order in which we store the engines inside default_engines() for the
legacy ctx->engines[] has to match the legacy I915_EXEC_RING selector
mapping in execbuf::user_map. If we present VCS2 as being the second
instance of the video engine, legacy userspace calls that I915_EXEC_BSD2
and so we need to insert it into the second video slot.

v2: Record the legacy mapping (hopefully we can remove this need in the
future)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111328


Fixes: 2edda80d ("drm/i915: Rename engines to match their user interface")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com> #v1
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190808110612.23539-2-chris@chris-wilson.co.uk