drm/i915/gtt: Relax assertion for pt_used

drm/i915/gtt: Relax assertion for pt_used

When inserting the final level PTE, we check that we are not overflowing
the page table (checking that pt_used does not exceed the size of the
table). However, we have to allow for every other PTE to be pinned by a
simultaneous removal thread (as on remove we bump the pt_used counter
before adjusting the table).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190821042044.7354-1-chris@chris-wilson.co.uk