drm/i915/gt: use new tasklet API for execution list

This converts the driver to use the new tasklet API introduced in
commit 12cc923f ("tasklet: Introduce new initialization API")

v2: Fix up selftests/execlists.

Signed-off-by: Emil Renner Berthing <kernel@esmil.dk>
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/20210126150155.1617-1-kernel@esmil.dk


Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>