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

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 4dd738e9cd79
("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 <[email protected]>
Cc: Chris Wilson <[email protected]>
Cc: Dave Gordon <[email protected]>
Cc: Ville Syrjälä <[email protected]>
Cc: Michel Thierry <[email protected]>
Signed-off-by: Mika Kuoppala <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>