drm/i915: Tidy up flush cpu/gtt write domains

Since we know the write domain, we can drop the local variable and make
the code look a tiny bit simpler.

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/20160818161718.27187-12-chris@chris-wilson.co.uk