drm/i915: Unconditionally initialise the interrupt workers

Rather than duplicate similar code across the IRQ installers, perform
the initialisation of the workers upfront. This will lead to simpler
teardown and quiescent code as we can assume that the workers have
been initialised.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>