drm/i915: add in-kernel blitter client

The plan is to use the blitter engine for async object clearing when
using local memory, but before we can move the worker to get_pages() we
have to first tame some more of our struct_mutex usage. With this in
mind we should be able to upstream the object clearing as some
selftests, which should serve as a guinea pig for the ongoing locking
rework and upcoming async get_pages() framework.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: CQ Tang <cq.tang@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/20190529123108.24422-2-matthew.auld@intel.com