drm/i915: Exclude low pages (128KiB) of stolen from use

The GPU is trashing the low pages of its reserved memory upon reset. If
we are using this memory for ringbuffers, then we will dutiful resubmit
the trashed rings after the reset causing further resets, and worse. We
must exclude this range from our own use. The value of 128KiB was found
by empirical measurement (and verified now with a selftest) on gen9.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@vger.kernel.org
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201019165005.18128-2-chris@chris-wilson.co.uk