drm/i915: Fix 32b overflow check in gen8_ppgtt_alloc_page_directories

drm/i915: Fix 32b overflow check in gen8_ppgtt_alloc_page_directories

The patch 69876bed7e008f5fe01538a2d47c09f2862129d0: "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 <[email protected]>
Cc: Dave Gordon <[email protected]>
Cc: Ville Syrjälä <[email protected]>
Signed-off-by: Michel Thierry <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>