We write all 4K page entries, even when using 64K pages. In order to
verify that the HW isn't cheating by using the 4K PTE instead of the 64K
PTE, we want to remove all the surplus entries. If the HW skipped the
64K PTE, it will read/write into the scratch page instead - which we
detect as missing results during selftests.

v2: much improved commentary (Chris)
Signed-off-by: NMatthew Auld <matthew.auld@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Changbin Du <changbin.du@intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180511095140.25590-1-matthew.auld@intel.com