drm/i915: Explicitly mark Global GTT address spaces

So far we have been relying on vm->file pointer being NULL to declare
something GGTT.

This has the unfortunate consequence that the default kernel context is
also declared GGTT and interferes with the following patch which wants to
instantiate VMA's and execute requests against the kernel context.

Change the is_ggtt test to use an explicit flag in struct address_space to
solve this issue.

Note that the bit used is free since there is an alignment hole in the
struct.

v2:
 * Mark mock ggtt.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180831143643.12366-1-tvrtko.ursulin@linux.intel.com