drm/i915: Allow for different modes of interruptible i915_active_wait

Allow some users the discretion to not immediately return on a normal
signal. Hopefully, they will opt to use TASK_KILLABLE instead.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200327112212.16046-1-chris@chris-wilson.co.uk