drm/i915/gt: Remove uninterruptible parameter from intel_gt_wait_for_idle

Now that the only user of the uninterruptible wait was eliminated,
remove the support.

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/20201209164008.5487-3-chris@chris-wilson.co.uk