Revert "Revert "drm/amdkfd: Add memory sync before TLB flush on unmap""

This reverts commit 4bba567c.

Revert reason: The issue has been resolved.

Signed-off-by: Eric Huang <jinhuieric.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>