drm/i915: Report context GTT size

drm/i915: Report context GTT size

Since the beginning we have conflated the size of the global GTT with
that of the per-process context sizes. In recent times (gen8+), those
are no longer the same where the global GTT is limited to 2/4GiB but the
per-process GTT may be anything up to 256TiB. Userspace knows nothing of
this discrepancy and outside of one or two hacks, uses the getaperture
ioctl to determine the maximum size it can use. Let's leave that as
reporting the global GTT and use the context reporting method to
describe the per-process value (which naturally fallsback to reporting
the aliasing or global on older platforms, so userspace can always use
this method where available).

Testcase: igt/gem_userptr_blits/minor-normal-sync
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=90065
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>