ttm: Clear the ttm page allocated from high memory zone correctly

The TTM page can be allocated from high memory. In such case it is
wrong to use the page_address(page) as the virtual address for the high memory
page.

bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=50241

Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Cc: stable@vger.kernel.org
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>