drm/i915: Report correct GGTT space usage

Currently only normal views were accounted which under-accounts
the usage as reported in debugfs.

Introduce new helper, i915_gem_obj_total_ggtt_size, and use it
from call sites which want to know how much GGTT space are
objects using.

v2: Single loop in i915_gem_get_aperture_ioctl. (Chris Wilson)

v3: Walk GGTT active/inactive lists in i915_gem_get_aperture_ioctl
    for better efficiency. (Chris Wilson, Daniel Vetter)

v4: Make i915_gem_obj_total_ggtt_size private to debugfs. (Chris Wilson)

v5: Change unsigned long to u64. (Chris Wilson)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>