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

It is to fix a failure for SDMA updating PTEs.

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