drm/i915: Prevent out of range pt in gen6_for_each_pde

Found by static analysis tool, this was harmless as the pt was not
used out of scope though.

Introduced by commit 678d96fb
("drm/i915: Track GEN6 page table usage").

Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>