drm/i915: Skip clearing the GGTT on full-ppgtt systems

Under full-ppgtt, access to the global GTT is carefully regulated
through hardware functions (i.e. userspace cannot read and write to
arbitrary locations in the GGTT via the GPU). With this restriction in
place, we can forgo clearing stale entries from the GGTT as they will
not be accessed.

For aliasing-ppgtt, we could almost do the same except that we do allow
userspace access to the global-GTT via execbuf in order to workraound
some quirks of certain instructions. (This execbuf path is filtered out
with EINVAL on full-ppgtt.)

The most dramatic effect this will have will be during resume, as with
full-ppgtt the GGTT is only used sparingly.

References: https://bugs.freedesktop.org/show_bug.cgi?id=94722
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: David Weinehall <david.weinehall@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Tested-by: David Weinehall <david.weinehall@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463207195-22076-4-git-send-email-chris@chris-wilson.co.uk