drm/amdkfd: fix TLB flush after unmap for GFX9.4.2

TLB flush after unmap accidentially was removed on
gfx9.4.2. It is to add it back.

Signed-off-by: Eric Huang <jinhuieric.huang@amd.com>
Reviewed-by: Harish Kasiviswanathan <Harish.Kasiviswanathan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org