drm/i915: Use the size/type of address space to make decisions

Once the address space has been created (using 3 or 4 levels of page
tables), we should use that to program the appropriate type into the
contexts. This gives us the flexibility to handle different types of
address spaces at runtime.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170209144036.23664-1-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>