drm/i915/selftests: add write-dword test for LMEM

Simple test writing to dwords across an object, using various engines in
a randomized order, checking that our writes land from the cpu.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191025153728.23689-4-chris@chris-wilson.co.uk