drm/i915: Take obj lock around set_domain ioctl

We need to lock the object to move it to the correct domain,
add the missing lock.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
[danvet: Pick version from an older patch series.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210128162612.927917-29-maarten.lankhorst@linux.intel.com
Link: https://patchwork.freedesktop.org/patch/msgid/20210323155059.628690-29-maarten.lankhorst@linux.intel.com