drm/i915/selftests: move gpu-write-dw into utils

Using the gpu to write to some dword over a number of pages is rather
useful, and we already have two copies of such a thing, and we don't
want a third so move it to utils. There is probably some other stuff
also...

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/20190810105008.14320-1-chris@chris-wilson.co.uk