drm/i915: introduce GEM_WARN_ON

In a similar spirit to GEM_BUG_ON we now also have GEM_WARN_ON, with the
simple goal of expressing warnings which are truly insane, and so are
only really useful for CI where we have some abusive tests.

v2:
  - use BUILD_BUG_ON_INVALID for !DEBUG_GEM
  - clarify commit message

Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161213203222.32564-1-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

@@ -27,8 +27,10 @@
 
 #ifdef CONFIG_DRM_I915_DEBUG_GEM
 #define GEM_BUG_ON(expr) BUG_ON(expr)
+#define GEM_WARN_ON(expr) WARN_ON(expr)
 #else
 #define GEM_BUG_ON(expr) BUILD_BUG_ON_INVALID(expr)
+#define GEM_WARN_ON(expr) (BUILD_BUG_ON_INVALID(expr), 0)
 #endif
 
 #define I915_NUM_ENGINES 5