drm/i915: WARN_ON failed map_and_fenceable

I just noticed in our code we don't really check the assertion, and
given some of the code I am changing in this area, I feel a WARN is very
nice to have.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: s/&/&&/ to fix typo on the check.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>