drm/i915/gtt: Fix the boundary check for vm area

The check for start + length >= total_vm_size is
wrong since start + length can be exactly the size of
the vm.

Fix the check to allow allocation to boundary.

Fixes a regression in commit 4dd738e9
("drm/i915: Fix 32b overflow check in gen8_ppgtt_alloc_page_directories")

Testcase: igt/gem_evict_everything/swapping-interruptible
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=90399


Tested-by: Lu Hua <huax.lu@intel.com>
Cc: Chris Wilson <chris@chris.wilson.co.uk>
Cc: Dave Gordon <david.s.gordon@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>