drm/i915: allow GEM_BUG_ON expr checking with !DEBUG_GEM

Use BUILD_BUG_ON_INVALID(expr) in GEM_BUG_ON when building without
DEBUG_GEM. This means the compiler can now check the validity of expr
without generating any code, in turn preventing us from inadvertently
breaking the build when DEBUG_GEM is not enabled.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161202184750.3843-1-matthew.auld@intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_gem.h

@@ -28,7 +28,7 @@
 #ifdef CONFIG_DRM_I915_DEBUG_GEM
 #define GEM_BUG_ON(expr) BUG_ON(expr)
 #else
-#define GEM_BUG_ON(expr) do { } while (0)
+#define GEM_BUG_ON(expr) BUILD_BUG_ON_INVALID(expr)
 #endif
 
 #define I915_NUM_ENGINES 5