drm/i915: use cpu_relax() in wait_for_atomic

drm/i915: use cpu_relax() in wait_for_atomic

As part of the advice given to us from the hardware designers regarding
the maximum wait time on the forcewake handshake we need to move from us
granularity to ms granularity. In earlier patches to do this, Jani
noticed that wait_for_us was properly converted to use cpu_relax(), but
wait_for was not.

The issue has existed since the introduction of the macro:
commit cd565a50c9be30afb9dcb58c6dbe50e9813ed7d2
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Sat Aug 7 11:01:35 2010 +0100

    drm/i915: Ensure that while(INREG()) are bounded (v2)

CC: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Chris Wilson <chris@chris-wilson.oc.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>