drm/i915/selftests: Compare user mmap against GPU

Check that the user writes into their mmap are visible on the GPU.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Acked-by: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200103204137.2131004-3-chris@chris-wilson.co.uk