drm/i915: Trim the flush for the legacy request emission

At the start of request emission, we flush some space for the request,
estimating the typical size for the request body. The tail is now much
larger than the typical body, so we can shrink the flush slightly.

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/1461917226-9132-2-git-send-email-chris@chris-wilson.co.uk