drm/i915/ringbuffer: Specialise SNB+ request emission for semaphores

As gen6_emit_request() only differs from i9xx_emit_request() when
semaphores are enabled, only use the specialised vfunc in that scenario.

v2: Reorder semaphore init so as to keep engine->emit_request default
vfunc selection compact.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1469432687-22756-27-git-send-email-chris@chris-wilson.co.uk


Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470174640-18242-18-git-send-email-chris@chris-wilson.co.uk