drm/i915: Unify legacy/execlists emission of MI_BATCHBUFFER_START

Both the ->dispatch_execbuffer and ->emit_bb_start callbacks do exactly
the same thing, add MI_BATCHBUFFER_START to the request's ringbuffer -
we need only one vfunc.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1469432687-22756-20-git-send-email-chris@chris-wilson.co.uk
Link: http://patchwork.freedesktop.org/patch/msgid/1470174640-18242-10-git-send-email-chris@chris-wilson.co.uk