drm/i915: Avoid overflow in computing pot_hole loop termination

drm/i915: Avoid overflow in computing pot_hole loop termination

When using the mock_ppgtt selftest, the GTT is large enough to cause an
overflow in pot_hole() when adding 2 pages to the address. Avoid the
overflow by computing the final valid address and iterating up to that
address.

Signed-off-by: Chris Wilson <[email protected]>
Cc: Matthew Auld <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Matthew Auld <[email protected]>