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>

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -756,8 +756,8 @@ static int gen8_ppgtt_alloc_page_directories(struct i915_hw_ppgtt *ppgtt,
 
 	WARN_ON(!bitmap_empty(new_pds, GEN8_LEGACY_PDPES));
 
-	/* FIXME: PPGTT container_of won't work for 64b */
-	WARN_ON((start + length) > 0x800000000ULL);
+	/* FIXME: upper bound must not overflow 32 bits  */
+	WARN_ON((start + length) >= (1ULL << 32));
 
 	gen8_for_each_pdpe(pd, pdp, start, length, temp, pdpe) {
 		if (pd)