drm/i915: Fix GCC 4.4 build issue with __intel_wait_for_register_fw

Move the BUILD_BUG_ONs for busy-wait duration outside the
_wait_for_atomic macro as discussed on the mailing list.

v2: Simplify the macro by omitting the ret__ local. (Chris Wilson)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Suggested-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Fixes: 1d1a9774e404 ("drm/i915: Extend intel_wait_for_register_fw() with fast timeout")
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170418105211.7089-1-tvrtko.ursulin@linux.intel.com

drivers/gpu/drm/i915/intel_drv.h

@@ -88,7 +88,6 @@
 	int cpu, ret, timeout = (US) * 1000; \
 	u64 base; \
 	_WAIT_FOR_ATOMIC_CHECK(ATOMIC); \
-	BUILD_BUG_ON((US) > 50000); \
 	if (!(ATOMIC)) { \
 		preempt_disable(); \
 		cpu = smp_processor_id(); \
@@ -130,8 +129,14 @@
 	ret__; \
 })
 
-#define wait_for_atomic(COND, MS)	_wait_for_atomic((COND), (MS) * 1000, 1)
-#define wait_for_atomic_us(COND, US)	_wait_for_atomic((COND), (US), 1)
+#define wait_for_atomic_us(COND, US) \
+({ \
+	BUILD_BUG_ON(!__builtin_constant_p(US)); \
+	BUILD_BUG_ON((US) > 50000); \
+	_wait_for_atomic((COND), (US), 1); \
+})
+
+#define wait_for_atomic(COND, MS) wait_for_atomic_us((COND), (MS) * 1000)
 
 #define KHz(x) (1000 * (x))
 #define MHz(x) KHz(1000 * (x))