drm/i915: Use engine to refer to the user's BSD intel_engine_cs

drm/i915: Use engine to refer to the user's BSD intel_engine_cs

This patch transitions the execbuf engine selection away from using the
ring nomenclature - though we still refer to the user's incoming
selector as their user_ring_id.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1469432687-22756-7-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1469606850-28659-2-git-send-email-chris@chris-wilson.co.uk