On most gen2-4 platforms the GTT can be (or maybe always is?)
inside the stolen memory region. If that's the case, reduce the
size of the stolen memory appropriately to make make sure we
don't clobber the GTT.

v2: Deal with gen4 36 bit physical address
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=80151Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>