drm/i915: avoid harmless empty-body warning

The newly added assert_kernel_context_is_current introduces a warning
when built with W=1:

drivers/gpu/drm/i915/i915_gem.c: In function ‘assert_kernel_context_is_current’:
drivers/gpu/drm/i915/i915_gem.c:4417:63: error: suggest braces around empty body in an ‘else’ statement [-Werror=empty-body]

Changing the GEM_BUG_ON() macro from an empty definition to "do { } while (0)"
makes the macro more robust to use and avoids the warning.

Fixes: 3033acab ("drm/i915: Queue the idling context switch after all other timelines")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161108135834.2166677-1-arnd@arndb.de

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)
+#define GEM_BUG_ON(expr) do { } while (0)
 #endif
 
 #define I915_NUM_ENGINES 5