drm/i915: Drop the msleep parameter to wait_for()

Jesse's feedback from using the wait_for() macro was that the msleep
argument was that it was superfluous and made the macro more difficult
to use and to read. As the actually amount of time to sleep is not
critical, the crucial part is to sleep and let the processor schedule
something else whilst we wait for the event, replace the argument with a
hardcoded value.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>