drm/ttm: Fix vma page_prot bit manipulation

Fix a long-standing TTM issue where we manipulated the vma page_prot
bits while mmap_sem was taken in read mode only. We now make a local
copy of the vma structure which we pass when we set the ptes.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>