drm/i915: dma_buf_vunmap is presumed not to fail, don't let it

Since dma_buf_vunmap() procedes blithely on ignorant of whether the
driver failed to actually unmap the backing storage for the dma-buf, we
need to make a best-effort to do so. This involves not allowing
ourselves to be susceptible to signals causing us to leak the storage.

This should have been detectable with the current i-g-t as a misplaced
signal should have left the pages pinned upon freeing the object where
we have a warning in place.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>