Revert "drm/i915: Enable semaphores on BDW"

This reverts commit 521e62e4.

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

drivers/gpu/drm/i915/i915_drv.c

@@ -481,6 +481,10 @@ bool i915_semaphore_is_enabled(struct drm_device *dev)
 	if (i915.semaphores >= 0)
 		return i915.semaphores;
 
+	/* Until we get further testing... */
+	if (IS_GEN8(dev))
+		return false;
+
 #ifdef CONFIG_INTEL_IOMMU
 	/* Enable semaphores on SNB when IO remapping is off */
 	if (INTEL_INFO(dev)->gen == 6 && intel_iommu_gfx_mapped)