drm/i915: Remove chipset flush after cache flush

drm/i915: Remove chipset flush after cache flush

We always flush the chipset prior to executing with the GPU, so we can
skip the flush during ordinary domain management.

This should help mitigate some of the potential performance regressions,
but likely trivial, from doing the flush unconditionally before execbuf
introduced in commit 85be9286142f ("drm/i915: Unconditionally flush any
chipset buffers before execbuf")

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20161106130001.9509-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>