drm/i915: fold wait_for_atomic_us into wait_for_atomic

Since

commit bcf9dcc1
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Sun Jul 15 09:42:38 2012 +0100

    drm/i915: Workaround hang with BSD and forcewake on SandyBridge

and

commit 0cc2764c


Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Sat Sep 1 22:59:48 2012 -0700

    drm/i915: use cpu_relax() in wait_for_atomic

these two macros are essentially the same, so unify them. We keep the
_us version since it's a nice documentation for smaller timeouts.

v2: Fixup time unit conversion, _wait_for takes ms (Ville).

Cc: Jack Winter <jbh@alchemy.lu>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>