drm/i915: Make contexts non-snooped on non-LLC platforms

We don't do CPU access to GPU contexts so making the GPU access snoop
the CPU caches seems silly, and potentially expensive.

v2: Use !IS_VALLEYVIEW instead of HAS_LLC as this is really
    about what the PTEs can represent.
    Add a comment clarifying the situation.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>