drm/i915: Rework clflush to work correctly without obj->mm.lock.

Pin in the caller, not in the work itself. This should also
work better for dma-fence annotations.

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/20210323155059.628690-22-maarten.lankhorst@linux.intel.com