drm: writeback: Fix use after free in drm_writeback_connector_cleanup()

drm: writeback: Fix use after free in drm_writeback_connector_cleanup()

The drm_writeback_cleanup_job() function frees "pos" so call
list_del(&pos->list_entry) first to avoid a use after free.

Fixes: 1914ba2b91ea ("drm: writeback: Create drmm variants for drm_writeback_connector initialization")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/78abd541-71e9-4b3b-a05d-2c7caf8d5b2f@stanley.mountain
Signed-off-by: Maxime Ripard <mripard@kernel.org>