drm/i915: Only expand COND once in wait_for()

Commentary from Chris Wilson's original version:

> I was looking at some wait_for() timeouts on a slow system, with lots of
> debug enabled (KASAN, lockdep, mmio_debug). Thinking that we were
> mishandling the timeout, I tried to ensure that we loop at least once
> after first testing COND. However, the double test of COND either side
> of the timeout check makes that unlikely. But we can do an equivalent
> loop, that keeps the COND check after testing for timeout (required so
> that we are not preempted between testing COND and then testing for a
> timeout) without expanding COND twice.
>
> The advantage of only expanding COND once is a dramatic reduction in
> code size:
>
>    text	   data	    bss	    dec	    hex
>    1308733	   5184	   1152	1315069	 1410fd	before
>    1305341	   5184	   1152	1311677	 1403bd	after

but it turned out that due to a missing iniitialiser, gcc had "gone
wild trimming undefined code" :( This version acheives a rather more
modest (but still worthwhile) gain of ~550 bytes.
Signed-off-by: Dave Gordon <david.s.gordon@intel.com>
Original-idea-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Zanoni, Paulo R <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1473855033-26980-1-git-send-email-david.s.gordon@intel.comReviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/intel_drv.h

@@ -52,11 +52,15 @@
  */
 #define _wait_for(COND, US, W) ({ \
 	unsigned long timeout__ = jiffies + usecs_to_jiffies(US) + 1;	\
-	int ret__ = 0;							\
-	while (!(COND)) {						\
-		if (time_after(jiffies, timeout__)) {			\
-			if (!(COND))					\
-				ret__ = -ETIMEDOUT;			\
+	int ret__;							\
+	for (;;) {							\
+		bool expired__ = time_after(jiffies, timeout__);	\
+		if (COND) {						\
+			ret__ = 0;					\
+			break;						\
+		}							\
+		if (expired__) {					\
+			ret__ = -ETIMEDOUT;				\
 			break;						\
 		}							\
 		if ((W) && drm_can_sleep()) {				\