intel: Detect cache domain inconsistency with valgrind

Every access to either the GTT or CPU pointer is supposed to be
proceeded by a set_domain ioctl so that GEM is able to manage the cache
domains correctly and for the following access to be coherent. Of
course, some people explicitly want incoherent, non-blocking access
which is going to trigger warnings by this patch but are probably better
served by explicit suppression.

v2: Also mark the pointers as inaccessible following the explicit unmap
and implicit unmap upon return to the cache.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>