drm/i915: Use the same vfunc for BSD2 ring init

drm/i915: Use the same vfunc for BSD2 ring init

If we needed to do something different for the init functions, we could
always look at the engine instance to make the distinction. But, in any
case, the two functions are virtually identical already (please notice
that BSD2_RING is only used from gen8 onwards).

With this, the init functions depends excusively on the engine class
(a fact that we will use soon).

v2: Commit message

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1491834873-9345-3-git-send-email-oscar.mateo@intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>