drm/i915: kill set_need_resched

This is just a remnant from the old days when our reset handling was
horribly racy, suffered from terribly locking issues and often happily
live-locked. Those days are now gone so we can drop the hacks and just
rip the reschedule-point out.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>