drm/i915: Fix 32b overflow check in gen8_ppgtt_alloc_page_directories

The patch 69876bed: "drm/i915/gen8:
page directories rework allocation" added an overflow warning, but the
mask had an extra 0. Use less typo-prone option suggested by Dave
instead, to check for (start + length) >= 0x100000000ULL.

This check will be unnecessary after gen8_alloc_va_range handles more
than 4 PDPs (48b addressing).

v2: Really check for 32b overflow (Ville)

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Dave Gordon <david.s.gordon@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>