drm/i915: make caching operate on all address spaces

For now, objects will maintain the same cache levels amongst all address
spaces. This is to limit the risk of bugs, as playing with cacheability
in the different domains can be very error prone.

In the future, it may be optimal to allow setting domains per VMA (ie.
an object bound into an address space).

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>