drm/i915: Update comment before i915_spin_request

~jiffie and a few usecs is 3 orders of magnitude different. A bit
much. This was changed in

commit ca5b721e
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Fri Dec 11 11:32:58 2015 +0000

    drm/i915: Limit the busy wait on requests to 5us not 10ms!

But probably missed the comment since the change was non-local to the
comment.

v2: Polish comment more (Chris).

Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470413484-23775-1-git-send-email-daniel.vetter@ffwll.ch