When advancing onto the next 4th level page table entry, we need to
reset our indices to 0. Currently we restart from the original address
which means we start with an offset into the next PML table.

Fixes: 894ccebe ("drm/i915: Micro-optimise gen8_ppgtt_insert_entries()")
Reported-by: NMatthew Auld <matthew.william.auld@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99948
Testcase: igt/drv_selftest/live_gtt
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Tested-by: NMatthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: NMatthew Auld <matthew.william.auld@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170225181122.4788-4-chris@chris-wilson.co.uk