gma500: mmu: unmap the correct address

The function kunmap_atomatic() is called on the same variable twice,
i.e. pt->v. In the second call, its parameter should be variable v
rather than pt->v.

Signed-off-by: Pan Bian <bianpan2016@163.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1509256512-5962-1-git-send-email-bianpan2016@163.com