drm/i915/selftests: Prepare igt_gem_utils for obj->mm.lock removal

igt_emit_store_dw needs to use the unlocked version, as it's not
holding a lock. This fixes igt_gpu_fill_dw() which is used by
some other selftests.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210323155059.628690-51-maarten.lankhorst@linux.intel.com