drm/ttm: remove unneeded preempt_disable/enable

It is unnecessary to disable preemption explicitly while calling
copy_highpage().  Because copy_highpage() will do it again through
kmap_atomic/kunmap_atomic.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>