drm/i915/gt: Stop cross-polluting PIN_GLOBAL with PIN_USER with no-ppgtt

drm/i915/gt: Stop cross-polluting PIN_GLOBAL with PIN_USER with no-ppgtt

In order to keep userptr distinct from ggtt mmaps in the eyes of
lockdep, we need to avoid marking those userptr vma as PIN_GLOBAL. (So
long as we comply with only using them as local PIN_USER!)

References: https://gitlab.freedesktop.org/drm/intel/-/issues/1880
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200525075347.582-1-chris@chris-wilson.co.uk