Revert "drm/i915: Enable semaphores on BDW"

Revert "drm/i915: Enable semaphores on BDW"

This reverts commit 95ea3b7c02ad69f7e8ffb6faf3c711087547869e.

Although POST_SYNC brought a bit of stability to Semaphores on BDW
it didn't solved all issues and some hungs can still occour when
semaphores are enabled on BDW. Also some sloweness can be found on some
igt tests, althoguth it apparently doesn't affect real workloads.

Besides that, no real performance gain was found on our tests with different
and even multiple workloads.

Let's disable it again for now. At least until we are sure it is safe
to re-enable it.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>